I got a free large Pelican-TYPE case (not sure if it is Pelican brand) because it has a corner that was cracked from being tenderly loved by airline baggage handlers. The crack at the corner is about 1.5" x 1.5" L-shaped. Otherwise the case is in great condition other than a few scuffs. I would like to restore the waterproof integrity of the case without spending a bunch of money or creating a big mess. What product/process would you recommend to fix or patch the crack?

Waterproof Marine Epoxy.

Call Pelican, their cases have a lifetime warranty, regardless if sold second hand.  

They have provided replacement latches free of charge for old cases that I have.

ETa, reread your post, if not Pelican they won't help you.  Otherwise, marine epoxy as described above.

Yep, Pelican has absolutely amazing warranty support, but "pelican like" does not.  They are expensive but you get what you pay for.  If it's a brand name case like stormcase you may still be able to get warranty help.