Ended up moving over to generic events since the GLX type code is part of the GLX namespace and larger than 8 bits.
Apparently no one had ever tried ChangeDrawableAttributes with indirect clients, because simply going a glXSelectEvent causes a crash in that case. So this patch set includes a fix for that issue and has been tested with both direct and indirect clients, both with and without client and server support.
New clients with old servers should get a 0 sbc rather than garbage. New servers will send old clients a 0 sbc, but the clients will still copy garbage into the event actually sent to clients. New servers with new clients should work both in the direct and indirect cases and get whatever values the server sends for each field.
As always, testing and review appreciated. Piglit's glx-swap-event case now checks for swap count sanity (i.e. increasing, nonzero value) and allows you to dump the current values in verbose mode. I also added a few more return value checks to the test.
Thanks, Jesse