The problem there is it's only "WhatLinksHere" in English. Other languages use localised versions, so it would be a massive regex very prone to breakage...
Hmm, couldn't you pull the localized name(s) from $specialPageAliases ? Check the wiki's language, find the right alias and chop that off.
That could work, but after looking through the code I found a function in SpecialPageFactory that does this already! (Except instead of looking up the page name it cuts everything off up until the first forward-slash.)
$titleParts = SpecialPageFactory::resolveAlias($wgTitle->getText()); $subpageText = $titleParts[1];
This seems to work well.
Thanks for your help!
Cheers, Adam.