XML Namespace for Experimental Features

Question

We've hard forked the XML schema AIXM 4.5 to properly add our up-to-then informal extensions as OFMX. This is finished now and we'll release 1.0 of OFMX soonish.

From here on forward, we'd like to implement future extensions in a separate namespace and (after an extension has proven useful) move it to the main schema as a new release.

Unfortunately, AIXM (and OFMX for that matter) is imposing a strict order on the nodes which seems to complicate things. Here's an example:

<?xml version="1.0" encoding="UTF-8"?>
<OFMX-Snapshot xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="http://schema.openflightmaps.org/0.1/OFMX-Snapshot.xsd" version="0.1" origin="example" namespace="00000000-0000-0000-0000-000000000000" regions="LF" created="2018-01-01T12:09:15Z" effective="2018-01-04T00:00:00Z" expiration="2018-02-01T00:00:00Z">
  <Ser source="LF|AD|1.6|2018-01-04|1846">
    <SerUid mid="6a3ceb1b-5bbf-1fe4-8926-6fddd6bd1169">
      <UniUid region="LF" mid="d105b6b6-e020-2a73-5ec9-5eb3d5d86499">
        <txtName>STRASBOURG</txtName>
        <codeType>APP</codeType>
      </UniUid>
      <codeType>APP</codeType>
      <noSeq>1</noSeq>
    </SerUid>
    <Stt>
      <codeWorkHr>TIMSH</codeWorkHr>
      <Timsh>
        <codeTimeRef>UTCW</codeTimeRef>
        <dateValidWef>01-01</dateValidWef>
        <dateYearValidWef>2019</dateYearValidWef>
        <dateValidTil>31-12</dateValidTil>
        <dateYearValidTil>2020</dateYearValidTil>
        <codeDay>MON</codeDay>
        <codeDayTil>FRI</codeDayTil>
        <timeWef>08:00</timeWef>
        <codeEventWef>SR</codeEventWef>
        <timeRelEventWef>-30</timeRelEventWef>
        <codeCombWef>L</codeCombWef>
        <timeTil>20:00</timeTil>
        <codeEventTil>SS</codeEventTil>
        <timeRelEventTil>30</timeRelEventTil>
        <codeCombTil>E</codeCombTil>
      </Timsh>
      <txtRmkWorkHr>Consult NOTAM</txtRmkWorkHr>
    </Stt>
    <txtRmk>aka STRASBOURG approche</txtRmk>
  </Ser>
</OFMX-Snapshot>

My probably naïve idea is to add a pre namespace like so:

<OFMX-Snapshot 
  xmlns:pre="http://schema.openflightmaps.org/1.0-pre/OFMX-Experimental.xsd"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
  xsi:noNamespaceSchemaLocation="http://schema.openflightmaps.org/0.1/OFMX-Snapshot.xsd"
  (...)
>

And then add experimental features like so:

<Stt>
  <pre:foo>bar</pre:foo>
  <codeWorkHr>TIMSH</codeWorkHr>
  (...)
</Stt>

Validating this results in:

element foo: Schemas validity error : Element '{http://schema.openflightmaps.org/1.0-pre/OFMX-Experimental.xsd}foo': This element is not expected. Expected is ( codeWorkHr ).

Is such a namespace approach for experimental features feasible at all given the way AIXM (and by extension OFMX) is built?

Thanks for your hints!