Me, I'm tempted to do a Ranma/Darkover story

Ranma/Darkover?  Millenia different in time, plus space, tone, etc, etc?  I'm
not sure why you're even tempted...

But, if you just want a cameo appearance, there are alternatives.  You can't be
(successfully) sued for copyright violation or trademark violation with even a
thin disguise on your setting and characters, thin enough so that most readers
will know who and what you're talking about.  For instance, if you invented a
planet called "Lightover" or "Darkunder", with a medieval society ruled by a
caste of telepaths, and avoid direct copying of any of MZB's characters' names
or place names, you couldn't be sued.  The standard of proof for copyright
infringement cases is extremely high.

You also can't be sued for "modest quotations for review purposes", another out
for short crossovers.

Neither of these would work particularly well for long crossovers, the former
for aesthetic reasons rather than legal (presumably you want to use the real
names if you're doing that amount of work).