Rustle
In my opinion to ban your twin from grass totaly is dangerous in terms of airmanship and leagal liability and I shall outline why.
Some time back a low time twin pilot landed an aircraft on a hard runway in a crosswind that was outside limmits , the result was a lot of damage. When asked why he did not use the into wind grass runway the reply was " twins Can't land on grass".
A landing on the grass in this case would have been uneventfull with a touchdown speed of about 40kt groundspeed !.
This some clubs would have you beleave is the truth and it sets the low time pilots up for an accident.
I can see why people are reluctant to let some types use grass runways and for this reason I would advise the club rules to read something like this:- " This aircraft should not be opperated from grass runways unless airsafety or good airmanship dictate otherwise".
If you put additional limitations on an aircraft that lead the low time pilot into an accident then you can bet some legal type will be on your case.
Don't let it happen to you !.