Alright, here's what I got, but it didn't work. Yes, I even tried +host_timescale .1 and -host_timescale 1, just to make sure it wouldn't work.
unbind mouse3
alias "slomo_t" "slomo_on"
alias "slomo_on" "alias slomo_t slomo_off; host_timescale .1"
alias "slomo_off" "alias slomo_t slomo_on; host_timescale 1"
bind "mouse3" "slomo_t"
Ninjaedit: Also, is there a Source code forum? I'd probably be a lot better there.
Ninjaedit2: Osht, forgot Sv_cheats 1. One minute.
Edit3: Holy god, it worked. Thanks! If anyone would like to explain why, though, that'd be great. Educational purposes.