I have attached a patch to allow org-link-display-parameters (introduced
in a previous patch for custom faces) to also provide a custom tooltip.
It can be a string or a function, and if neither the old behavior is
used.

WDYT?

-- 
Professor John Kitchin
Doherty Hall A207F
Department of Chemical Engineering
Carnegie Mellon University
Pittsburgh, PA 15213
412-268-7803
@johnkitchin
http://kitchingroup.cheme.cmu.edu