This is a little off the OP's topic, but I put a 20x4.25 tire in the rear. I don't think a narrower bottom bracket would work as it already has troubles with clearance around the tire to get into the lowest gears. Of course a little chain/tire rub doesn't seem to be a huge problem either (no knobs).

If I was doing the bike again, it probably wouldn't get the 20x4.25 tire, but even with that, I'd add an idler pulley system to allow standard Q-Factor cranks.

Not all the cranks are the same, and I think some of the vintage cranks use straighter crank arms, and potentially a narrower Q-Factor, so I might be able to save a small amount of width.

Another issue is that I normally set my cleats to the inside to push the shoes to the outside and minimize rub. I did the opposite on one occasion for the cargo bike, but wouldn't want to change the cleat position every time I changed bikes.

Anyway, back on topic, there may be some cranks like vintage Campy/Dura Ace cranks with straight arms that could potentially work better with crank shorteners than other modern cranks, potentially reducing the extra width of the shorteners somewhat. Also moving one's cleats to the outside of one's pedals and bring the shoes closer.

Depending on the crank design, it may also be possible to double or triple drill the cranks, although many cranks have a taper and hollowed parts that would make double and triple drilling cumbersome.

However, something like this White Industries crankset could likely have multiple pedal locations drilled and tapped.

White Industries Triple MTB Crankset Old School Custom Made 42/32/22T 175mm Vtg | eBay

Jumps may be large though. 20mm? 175/155/135?

There are also several adjustable PowerCrank models, but they give you independent right and left crank arms which may or may not be as desired. Maybe a few other adjustable cranks too.