To my credit, Spellscape posted that his crank is a cottered crank after my post.

That said, the cottered crank might have an advantage...the only on cottered cranks have...over a three piece crank. The bottom bracket tool could be held in place with a stack of washers using the cottered arm to hold the stack and tool in place. The cotter pin could be placed loosely in the arm to hold every in place.

In fact, that might be an improvement over my method. Instead of using an old cup to hold it the tool and washers in place, a stack of fender washers would be all that would be needed. Put the tool on, put the washers on, and then put on the crank arm. The non-driveside arm could be used. Lightly bolt it in place and everything is held in place without any extra parts other than the fender washers.

I might even try making the spacer from the upper threaded part of a kickstand.