Instead of a script that download "on demand" the necessary PEAR libraries, things could be simpler: just put them as requirements. Is not something new, you must see what requires typical perl programs, and in the plus side, by this time (or at least the versions that tiki requires) there are enough of PEAR to at the very least being very automated or easy for the end user to download the needed modules..
From what I saw from tiki so far, it's common habbit to grab the user's hand to do anything. Having a script do actually fullfill those dependencies can't be bad.
If we're just to ask requirements, there are multiple extensions we could use to do most of the dirty work.
I don't see limitation. LGPL can be used in GPL projects.
Can you elaborate?