There is a city beach/park on the north end of Holmes beach/Longboat. It's pretty small, but could give at least the kayakers a good place to start (and either route to the Gulf or down the intercoastal depending on whether they head left or right).

Beach access on Longboat is difficult for boats, but the "sand Bar" has a street on the south side that I believe you are able to walk on to the beach from. Perhaps a fat beach wheeled boat could roll on the beach from there.

Don't the rules require you to be able to push your boat into the water? In that case I would presume most of the craft can be rolled out to the waterline (somehow) and might be able to get launched.

From the photos, it looks like a good multihull turnout. Was that red trimaran a Trailer-Tri (circa 1987)?