Alright, this feature has been implemented.
HUGE thanks to Cazic.Sphynx and Cazic.Sirenea for spending countless hours flagging spells by expansion, you guys are awesome!
I made some pretty hefty backend changes to the spell search page as well (at some point I'd really like to do a ground up rewrite of this script), so if you see something not working quite right, please let me know!