Bug#324179: Free content
You're right, there is no completely standalone Free game data for Quake 3.
However, this has been gotten around fairly simply before:
quake2 in contrib depends on quake2-data
quake2-data (also in contrib) presents a debconf question, allowing you
to download and use data files from the Quake 2 shareware, or install
the full data from a valid retail CD. Contrib is the right place for
this kind of software, and the use of an empty -data package with clever
debconf trickery (see: msttcorefonts) avoids the need for large non-free
chunks of data to reside on debian servers.