Who currently owns the Cadillacs & Dinosaurs/Xenozoic Tales license? (Or, how can I find out who does?)
The image below is from the first printing of the comic series; Xenozoic Tales. As you can see, the individual copyright credit for the panels found in the comics goes to artist Mark Schultz but the rights-holders for the Xenozoic Tales trademark (and presumably any ongoing use of the work) was Kitchen Sink Press, Inc.
According to the wikipedia article on Kitchen Press, the company went bankrupt in 1999 and was resurrected under the heading of Denis Kitchen Press Inc in 2001. Since then, they've appear to have held (and asserted) the rights on a continuous basis.
You can find their address and contact details here.