Well making some changes to the email channel (found in $CP_WEBINF/cars/email.car) to have the links take you into Horde/IMP I came across the following XSL extension that sungard has created.
cpExtension:formURL(aString)
What this does is create a URL on the Luminis server. You pass it the path to the file/script/image/etc and it will but in the appropriate http://servername/ piece at the start. The path you include needs to start from the root of the server, so no relative pathing.
It seems there are a number of options under the cpExtension umbrella too bad they are not documented.


Comments
Good find. Any idea if this
Good find. Any idea if this can be used in nested-tables.xsl? Also, could you post a sample of how you're using it?
Re: Nested-tables possibly
In the XSL that I found this in (emailchannel.xsl from email.car) they had the following in the xsl:stylesheet element indicating the various additional name spaces:
xmlns:cp="http://www.campuspipeline.com"
xmlns:cpExtension="com.pipeline.web.WebUtil"
xmlns:cpInfoExtension="campuspipeline.uportal.dlm.provider.ExtensionClass"
extension-element-prefixes="cpExtension cpInfoExtension"
exclude-result-prefixes="cpExtension cpInfoExtension"
So in theory you could try adding that to the xsl:stylesheet element in nested-tables.
Then to use it I am doing this (once again from the emailchannel.csl)
<xsl:value-of select="cpExtension:formURL(string('/cp/ip/login'))"/>
which will output http://luminis/cp/ip/login if I am currently on the domain luminis.