a device for turning coffee into software
The WikiPedia page on InterWiki has a link to the W3C draft specification Currie Syntax 1.0. This is not as powerful as some wiki engines provide, but it's an interesting approach.

Basically it defines a qname-like formatted reference inside square brackets that has its prefix replaced with the current namespace URI of the prefix. No prefix resolution mechanism is defined for non-XML documents.

As an example, consider the following fragment from an OGC web map context document:
<LayerList>
<Layer queryable="1" hidden="1" >
<Server service="WMS" version="1.1.1" title="World Map">
<OnlineResource type="simple"
xlink:href="http://www2.demis.nl/wms/wms.asp?wms=WorldMap"/>
</Server>
<SRS>EPSG:4326</SRS>
<Name>Ocean features</Name>
<Title>Ocean features</Title>
</Layer>
</LayerList>

Instead of using the full URL for the Demis WMS server, you could do something like the following:
<LayerList xmln:demis="http://www2.demis.nl/wms/wms.asp?wms=">
<Layer queryable="1" hidden="1" >
<Server service="WMS" version="1.1.1" title="World Map">
<OnlineResource type="simple"
xlink:href="[demis:WorldMap]"/>
</Server>
<SRS>EPSG:4326</SRS>
<Name>Ocean features</Name>
<Title>Ocean features</Title>
</Layer>
</LayerList>


So now, if you have 20 layers from Demis and you need to change the url, you can do it in one place.

One limitation to this approach is that you must still bind the URL to the prefix inside the document. There are many cases when you would like to have the prefix mapping stored in a database that can be edited by the user. If the XML is being generated dynamically, then this isn't a problem. However, if the XML is being generated dynamically then why not simply replace the URL when you generate the XML.