Thanks, gentlemen.
Interesting how a couple of opinions mention the importance of the
location. Am in San Francisco, but even if the local authority is
'easy' to deal with about a heavily modified craft; this brings the
question what reaction would sailing to a foreign port authorithy
bring, for a US registered monster, if they have to swallow it, or can
forbid it from sailing...

It would be a not 'for hire' craft; which may ease the insurance part
and taxes. There will be no intention to sell at all... actually
nobody would want such animal.

The tempting magnitude of lunatism could be an aircraft bolted onto a
registered catamaran (no mast); leaving room for an insurance inspector
to say "Nope. Cannot insure that thing" That could be a problem. No
intention to fly, just to go quite fast never taking off.
(keeping the fire extinguishes they want to see, the lights they want
to see, the meticulous location of the regitration numbering, life
vests...)

Well, will digest more about lunatisms...