I accepted an answer to the question as I asked it, but I'm adding this to show an alternate approach for a slightly different requirement. I needed to add a nested tag along with the text, which Sparkup does not support. So I found a single 2-step solution to this that also solved the original item number problem (more complex in my real world solution but simplified here).
This sparkup:
ion-content > .list > a.item.item-icon-left.Item$[href=#/items/$]*3 > i.icon.ion-email
generates this:
<ion-content>
<div class="list">
<a href="#/items/1" class="item item-icon-left Item1">
<i class="icon ion-email"></i>
</a>
<a href="#/items/2" class="item item-icon-left Item2">
<i class="icon ion-email"></i>
</a>
<a href="#/items/3" class="item item-icon-left Item3">
<i class="icon ion-email"></i>
</a>
</div>
</ion-content>
Then after I select the lines in visual mode, running this:
:'<,'>s/ Item\([0-9]*\)">/">Item \1/g
produces my desired result:
<ion-content>
<div class="list">
<a href="#/items/1" class="item item-icon-left">Item 1
<i class="icon ion-email"></i>
</a>
<a href="#/items/2" class="item item-icon-left">Item 2
<i class="icon ion-email"></i>
</a>
<a href="#/items/3" class="item item-icon-left">Item 3
<i class="icon ion-email"></i>
</a>
</div>
</ion-content>