Indeed, it was a cryptsy wallet issue but there is a simple solution - generate a new GLX deposit address on cryptsy and all transactions to the new address were processed smoothly today.
I wish I'd re-read the last part of this thread first - I sent a small test tx first, then the full balance once I saw it present and confirming.
I sent GLX to the old/existing address, so of course 562 confirms later it is still stuck and not credited.

edit: FWIW, after a bit of back and forth with first level support the ticket I opened only 90 mins ago has been escalated. Credit where credit is due.
edit 2: perhaps I should revise my praise slightly. Initial response was swift, but 5 days later I'm still waiting for the "technical specialists" to action my deposit.