The GUI problems will be fixed in v0.4.0 as the GUI (and its database backend) are being completely rewritten and this time I will test it on OSX as well.
As far as I know it should be possible to install libprs500 in any normal folder of your filesystem Each time the GUI is run it checks if the symlinks exists and point to the correct location, and if they don't it asks for the password.
The reason I chose /usr/bin is that /usr/local/bin is not in the PATH in a default installation of OS X.
libprs500.app is fully self-contained and should not require any external dependencies.
I suspect what's happening is that your installation of python 2.5.1 is interfering, though I don't see why that should happen.
Is the file libprs500.app/Contents/Resources/html2lrf.py being created when you run html2lrf?
EDIT: Also try cleaning everything out and re-installing libprs500. Remove /usr/bin/html2lrf, /usr/bin/prs500, /usr/bin/txt2lrf, /usr/bin/lit2lrf, /usr/bin/libprs500, /usr/bin/rtf-meta, /usr/bin/lrf-meta.
The delete libprs500.app and re download it. If it still doesn't work please post the output of
ls -l /usr/bin/html2lrf