I agree that shimming behind the bearing would be a better option, if you knew about the end-play issue ahead of time. With the outer shim method, I'm a little concerned that the bearing retainer could rub on that shim. It's unclear from the photos whether or not the bearing retainer clears the shim.
That retainer just floats in there, keeping the rollers aligned, and it spins at half the speed of the inner race (so not super fast, in a wheel bearing application). If it can still float freely with the shim installed, then you're probably okay, but if there's any pressure from the shim on the bearing retainer, you could have some metal scraping going on in there.
I know you know what you're doing though, so it's probably a non-issue. I love watching this thread!
You might be right. That shim might be touching the inner roller retainer/cage. I can't remember if when the inner bearing is fully seated in the outer race, IF the inner bearing cage is flush to the outside like the outer race, or if it sits a hair inward.
When I mocked it up, without the external seal in the way, I could turn the axle shaft, bearing would spin, the shim was held stationary because it is pinned between outer race and the brake backing plate.
I will grab an extra bearing from NAPA and experiment with an extra shim and see if there is clearance, or if the inner cage will rub on the backside of the shim.
I don't think it does, or if it does, it is negligible? If it is rubbing, I guess the force against the shim could try to twist the shim, tearing it to pieces, and making it become metal shrapnel inside the bearing, and causing excessive free play in both axle bearings.
I will investigate some more and report back