I can only think of Look’s adjustable cranks as an option. But they certainly didn’t have 20mm adjustability.

CC

I know they are out there because my son had a Trek that had a crank like this. I tried looking quickly but I couldn't combine any search word combo's to find such a thing. It was a kids model and I know the crank was not of any decent quality.

Adapter kits to let small fry ride on the backs of tandems sometimes include crank arms drilled for pedals at various lengths. I might have seen them among the recumbent crowd as well.

Your other alternative as you say is to have a second set of pedal holes drilled into a longer crank. This probably the most elegant solution. Only issue you might run into is whether there is enough difference in your desired lengths (seems that way to me, but the person drilling the cranks is the person you would have to convince).

Here you go. You're welcome.

Basic Powercranks

Yea, that's ~$900. Waaaay more then I want to spend on this.

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Strava

The concept is what I'm looking for, but it looks like it is 120mm and 140mm... Oh well, thanks anyway.

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Strava

Thanks. I'll look into the adapter kits. Never seen them before.

I think drilling a second hole in 172.5 cranks may be my best bet. The holes should fit, I just can't find a crank with enough material between the pedal hole and the bottom bracket to do it...

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Strava

What about getting a 2nd set of pedals and just swapping cranks in and out? That is super easy... not too different than moving the pedals.

You're evil.

While that would work in most cases, I use P2 pedals, and don't have it in my budget for another set.

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Strava

You could buy two cranks and then simply remove and install the pedals as needed. A little kore time consuming but fairly easy to do.