In the monohull world, YCs host events. They are able to host mixed-class events without upsetting the individual class associations or manufacturers, since they probably don't rely on them as heavily to support the events.
Too bad the YC aren't interested in hosting a beach cat regatta (regional/national level or higher). From what you say, they could get multiple box-rule and SMOD groups together at the same venue (and possibly in the same races - scored overall and SMOD), get multiple manufacture support/sponsorship, and certainly get interest generated in possible new members for the Yacht Club.
I guess if you were say, a Tiger, you'd have to choose whether to set up to race as SMOD or F18 when you registered, but those rock stars could probably register SMOD Tiger and still beat up on the "other" F18 fleet(s) to take home two pickle dishes...
