So I'm getting ready to start my warrior, and as I was leafing through the plans, I started to wonder about the feasibility of doing an extendable boom for the bottom bracket, as opposed to the clamp on bracket.

It would clean up the front end a bit, but you would lose some of the infinite adjustability.
You could use one size smaller square tubing that could slide into the boom tube.
Cut a keyed slot in the bottom and weld on a couple of pieces of small tubing on one side and some nuts on the other to clamp it down.
Or you could drill a series of say 1/4 inch holes every 2 inches or so for a span of about 10 inches at the end of the fixed boom and double pin the extendable boom into position.

I built a stinger boom lift for a forklift at a boatyard once that worked just like that. Although it was used for lifting up and down, not fore and aft.

To get a nice interference fit between the tubes, I'm thinking you would need to maybe sleeve the fixed boom tube with some UHMW plastic, or weld some very thin plate on the extendable portion and finesse it with a flap disc to get it to fit and slide properly.
Maybe a coat of some very thick paint on the sliding portion, for interference and abrasion. Like a bedliner material, or an epoxy paint.

Most of the factory tadpole trikes do this with the aluminum tubing of their booms.

Does that make any sense?
Maybe I need to dig up some visual aids.

Anybody have any other thoughts or ideas or maybe some definitive yays or nays?
Maybe its just too much extra work, for something that already works just peachy?