Sam, You highlight the rabbit hole in using the term "Class Legal" in a handicap race under ANY rating rule. It is a proven swamp!
Indeed, Deck sweepers are CLASS legal. They are not really an issue in a F18 race... However, once you start down that road... What's class... What's legal.... you wind up explaining F18s as yardsticks, why other ratings may or may not change, and on and on....
Show up for a handicap race against a deck sweeper F18, a "standard" rig and perhaps a Hobie tiger all called F18s. .... Questions will asked about the ratings used. .... God forbid you try to remember ALL of the flavors of the Nacra 6.0 as an OA for an event.... The class names are a trap.... They are a by product of US Dixie Portsmouth.... which is a rating system premised on FLEETS of sailors racing a Class of boat against ANOTHER fleet of sailors in a different class.
Best practice.... don't get into the class ID swamp..... or worse... the class legal briar patch... I think you simply deliver the message... Know your rating.
Most of the events, you don't need to present official paperwork. If you know that you are sailing a OD class legal Hobie 16.... you can honorably look up the H16 rating and report it. .... Its a number.... NOT a report of a class legal Hobie 16.... Likewise the owner of a Hobie 16 with a solid mast... Or a comp tip hobie 16 with non class legal customization.... just report your number... its the owners responsibility to figure it out or get some help. IF you are single handing your class legal Hobie 16...... KNOW your rating... and report it. Its a number! its the owners responsibility!
The "heavily modified Tornado" owner just needs to measure all the modifications etc and report the rating.... ITS a SINGLE NUMBER.... If asked... he can show his certificate that documents that simple Single number.
BTW.... Know your number! Its how all of the PHRF boats go racing... Hell PHRF scratch sheets may not even LIST the boat class.... they want yacht name, Sail number, captain name and your NUMBER....
QED.... Its the owners responsibility to KNow their Number!