About the closest I think you could get to that would be a multiple wet disk clutch.
It would require its own hydraulic system, and could be actuated with an electric proportional valve. It is possible that a dimensionally approximate clutch does exist in the forklift industry, or as an accessory drive clutch on very large machinery, like a modern transit bus air conditioning compressor. The TOWMOTOR in our shop uses one, and has a master cylinder to disengage it. You would certainly have to fabricate a custom input shaft, bellhousing, flywheel, crank adaptor plate, we're talking thousands here, and your first gear launch would be bone jarring.
I imagine though, that your intent is to be able to change gears without depressing a clutch pedal. You just cant do this the way manual gearboxes are designed. Basically a semi automatic transmission is an auto trans with individually selectable forward gears. Also known as, a powershift. It's much more feasible to start with an auto trans, and reverse engineer the valve body to give you individually selectable gears, than to start with a manual gearbox. In order to move from one gear to the next, you either have to interrupt power from the engine, or you have to mesh all gears together at once and put clutches on each one, so you can overlap the engagement. It's the only way to match input shaft speed with output shaft speed so the gears dont grind.
FWIW, I have driven graders and scrapers with powershift trannies, and theyre just as much fun to drive as their stickshift counterparts. And a lot easier on the leg, when you have so many forward gears
