I came up with the following, more generalized solution, after posting my original solution to the problem. The solution is pure XSLT and at the base, still uses <xsl:number>, so should be applicable to any format type.
<xsl:stylesheet version="2.0"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns:ewh="http://www.earlhood.com/XSL/Transform"
exclude-result-prefixes="#all">
<!-- Description: XSLT to generate a alpha formatted sequence
label (via <xsl:number>), but disallowing specific characters
from being used.
-->
<!-- Algorithm: Given the index value of the item to generate
a label for via <xsl:number>, we adjust the value so the resulting
label avoids the use of the forbidden characters.
This is achieved by converting the index value into a baseX
number, with X the number of allowed characters.
The baseX number will be converted into a reverse sequence
of numbers for each ^E place. For example, the number 12167
converted to base23 will generate the following reverse sequence:
Place: (23^0, 23^1, 23^2, 23^3)
Sequence: ( 0, 0, 0, 1) // 1000 in base23
Having it in right-to-left order makes processing easier.
Each item in the sequence will be a number from 0 to baseX-1.
With the sequence, we can then just call <xsl:number> on
each item and reverse concatenate the result.
NOTE: Since <xsl:number> does not like 0 as a given value,
the sequence must be processed so each item is within the
range of 1-to-baseX. For example, the above base23 example
will be translated to the following:
(23, 22, 22)
-->
<xsl:output method="xml" indent="yes"/>
<!-- Number of allowed characters: This should be total number of chars of
format-type desired minus the chars that should be skipped. -->
<xsl:variable name="lbase" select="23"/>
<!-- Sequence of character positions not allowed, with 1=>a to 26=>z -->
<xsl:variable name="lexcs" select="(9,12,15)"/> <!-- i,l,o -->
<!-- Helper Function:
Convert integer to sequence of number of given base.
The sequence of numbers is in reverse order: ^0,^1,^2,...^N.
-->
<xsl:function name="ewh:get_base_digits" as="item()*">
<xsl:param name="number" as="xs:integer"/>
<xsl:param name="to" as="xs:integer"/>
<xsl:variable name="Q" select="$number idiv $to"/>
<xsl:variable name="R" select="$number mod $to"/>
<xsl:sequence select="$R"/>
<xsl:if test="$Q gt 0">
<xsl:sequence select="ewh:get_base_digits($Q,$to)"/>
</xsl:if>
</xsl:function>
<!-- Helper Function:
Compute carry-overs in reverse-base digit sequence. XSLT starts
numbering at 1, so we cannot have any 0s.
-->
<xsl:function name="ewh:compute_carry_overs" as="item()*">
<xsl:param name="digits" as="item()*"/>
<xsl:variable name="d" select="subsequence($digits, 1, 1)"/>
<xsl:choose>
<xsl:when test="($d le 0) and (count($digits) = 1)">
<!-- 0 at end of list, nothing to do -->
</xsl:when>
<xsl:when test="$d le 0">
<!-- If digit <=0, need to perform carry-over operation -->
<xsl:variable name="next" select="subsequence($digits, 2, 1)"/>
<xsl:choose>
<xsl:when test="count($digits) le 2">
<xsl:sequence select="$lbase + $d"/>
<xsl:sequence select="ewh:compute_carry_overs($next - 1)"/>
</xsl:when>
<xsl:otherwise>
<xsl:sequence select="$lbase + $d"/>
<xsl:sequence select="ewh:compute_carry_overs(($next - 1,
subsequence($digits, 3)))"/>
</xsl:otherwise>
</xsl:choose>
</xsl:when>
<xsl:when test="count($digits) le 1">
<xsl:sequence select="$d"/>
</xsl:when>
<xsl:otherwise>
<xsl:sequence select="$d"/>
<xsl:sequence select="ewh:compute_carry_overs(subsequence($digits, 2))"/>
</xsl:otherwise>
</xsl:choose>
</xsl:function>
<!-- Helper Function:
Given a number in the base range, determine number for
purposes of <xsl:number>. We loop thru the exclusion
list and add 1 for each exclusion letter that has
been passed. The $digit parameter should be a number
in the range [1..$lbase].
-->
<xsl:function name="ewh:compute_digit_offset" as="xs:integer">
<xsl:param name="digit" as="xs:integer"/>
<xsl:param name="excludes" as="item()*"/>
<xsl:variable name="l" select="subsequence($excludes, 1, 1)"/>
<xsl:variable name="result">
<xsl:choose>
<xsl:when test="$digit lt $l">
<xsl:value-of select="0"/>
</xsl:when>
<xsl:when test="count($excludes) = 1">
<xsl:value-of select="1"/>
</xsl:when>
<xsl:otherwise>
<xsl:variable name="rest">
<xsl:value-of select="ewh:compute_digit_offset($digit+1,
subsequence($excludes,2))"/>
</xsl:variable>
<xsl:value-of select="1 + $rest"/>
</xsl:otherwise>
</xsl:choose>
</xsl:variable>
<xsl:value-of select="$result"/>
</xsl:function>
<!-- Retrieve alpha sequence label.
This is the main function to call.
-->
<xsl:function name="ewh:get-alpha-label" as="xs:string">
<xsl:param name="number" as="xs:integer"/>
<xsl:variable name="basedigits"
select="ewh:get_base_digits($number,$lbase)"/>
<xsl:variable name="digits"
select="ewh:compute_carry_overs($basedigits)"/>
<xsl:variable name="result" as="item()*">
<xsl:for-each select="$digits">
<xsl:variable name="digit" select="."/>
<!-- Should not have any 0 values. If some reason we do,
we ignore assuming they are trailing items. -->
<xsl:if test="$digit != 0">
<xsl:variable name="value">
<xsl:value-of select="$digit +
ewh:compute_digit_offset($digit,$lexcs)"/>
</xsl:variable>
<xsl:variable name="number">
<xsl:number value="$value" format="a"/>
</xsl:variable>
<xsl:sequence select="$number"/>
</xsl:if>
</xsl:for-each>
</xsl:variable>
<xsl:value-of select="string-join(reverse($result),'')"/>
</xsl:function>
<!-- For testing -->
<xsl:template match="/">
<result>
<xsl:for-each select="(1 to 1000,12166,12167,12168,279840,279841,279842)">
<value n="{.}"><xsl:value-of select="ewh:get-alpha-label(.)"/></value>
</xsl:for-each>
</result>
</xsl:template>
</xsl:stylesheet>
