I'm noticing that of the four versions of Web Grenade, Mastermind does not indicate that it accepts Slow enhancements while Blaster, Defender, and Corruptor do.
It's entirely possible that since the Traps power set is different for Masterminds than for Defenders and Corruptors (Detonator), that this is correct. My gut says it's probably not, though.
(Mace Mastery's Web Envelope and Web Cocoon also indicate that they accept Slow enhancements.)
If no one else gets a chance to check up on this one, I should be able to do so relatively quickly tonight by just jumping on the test server and creating a MM with Traps.