Since HP basically got out of the calculator business, the HP 50G, which in my opinion is a better calculator anyway, has been available to the public in software form for free. It uses the actual ROM code from the 50G, which HP donated to the public domain.
You have to look around a bit, but versions are available for Mac, PC, and Linux.
Got a link? I'm not seeing any evidence for your claim of public domain'd 50G code.
The closest I found was this uncited part of a Wiki article:
In 2003, the CAS source code of the 49G ROM was released under the LGPL. In addition, this release included an interactive geometry program and some commands to allow compatibility with certain programs written for the newer 49g+ calculator. Due to licensing restrictions, the recompiled ROM cannot be redistributed.