I should point out that Panda3D 1.4.0 now provides a StringStream class, which can be used either as an istream or ostream, solving this problem. setData() and getData() are the two primary Python interfaces.
In fact, most low-level Panda I/O calls now accept either an istream or a filename, making it possible to load just about any file from an in-memory StringStream instead of from a file on disk. You can even create a Multifile from a StringStream, and then mount it, thus making an entire file system from an in-memory image.
David