fixed space per set of URL s

Hi,

I'd like to assign a fixed amount of space to a certain set of URLs,
i.e. assign X MB for exclusive use for http://www.foo.org/* and do LRU
replacement within this 'partition'.
I can use refresh_pattern but this will only work until the cache starts
replacing fresh content by even fresher content (which is, given a certain
load level, quite likely).
Any ideas ? Other than running a separate Squid instance, of course.

Thanks,
Markus