Perhaps things are different in the UK, but I've never been in a position to supply a modeling pedal or head unless requested in the gig advance, and that's usually because the performers are traveling by commercial aircraft and need to hold down the size/weight of their baggage. Since this means the act will also need a drum kit, keyboards and other back line gear we get quotes from back line providers and tell the promoter or venue to take their pick of vendors and we're out of the provisioning game.
As a player I think I'd be taken aback by a local sound guy that said "here, you must use this." I suspect I'd say "up yours" and proceed to be a very difficult musician to work with. It's NOT your job to judge my art - whatever that may consist of - and any prejudicial attempt to control my work would be met with resistance.
If there is some kind of venue restriction it should be brought up in the booking/hiring process so the act has the option of declining the engagement or negotiating a variance.
Edit PS... if you find that you MUST use some external device, patch it console if possible. Back in the days of analog mixers with channel strip insert points it was an easy thing to do (I've replaced kick drums with an Alesis D2 sample, for instance). That said, I think the performer should have the initial and final say as to how he or she is presented. If the player is unconcerned, do what you think is necessary or proper...
