I did this by patching line 1410 in ISPConfig 2.2.24...works nicely.
However, there seems to be a little sideeffect:
Error-Pages are not shown anymore for these subdomains. If I just try to get a non-existant-link, I only get the standard error-page from IE. It doesn't matter to take my own error-pages or the once that come with ISPConfig. So this seems to be a little disadvantage with this patch...