SCHEMA compatibility with Oracle/DB2/Firebird

I know this isn't entirely postgresql specific, but it wouldn't be on
another list either so here goes...

I am writing an open source application where I would like to support
at least oracle, and possibly firebird or DB2, in addition to
postgresql which will be the default. I'm not going to try to support
mysql.

The application has many users, and in postgresql what works well is
to create a schema for each user instead of a separate database. The
main reason for schema's instead of databases is that the app runs
under mod perl, and there are too many users to have a pool of open
connections to each database.

There are also a set of common functions that I usually store in the
public schema. That way when working with the data of a particular
user I can do a SET search_path TO user,public, and have access to all
the functions without having to duplicate them in every schema.

My question is how easily would this work with other databases? I
know Oracle supports schema's, but I dont' know about the others. I
also don't know if other databases have the concept of a search path,
but I would think that they do.

Chris