I would like to offer my 2 cents. Take a look at the description for the following video, made in SFM of course:
http://www.youtube.com/watch?v=Q_ev32oOUxk
The maker of the video emailed Robin Walker himself, and got the following response:
"Thanks for sending this to me.
Fastest Kid in the World was great, btw.
Robin."
If Robin himself doesn't mind then it certainly shouldn't be a problem to have a version of sourcemod that works with it. Is it a problem that Sourcemod modifies locked cvars and can potentially alter the game? No, because Valve doesn't care. I think Source Filmmaker is similar.