We have a one-way UA booking for CTS-(ANA metal)-KIX-(UA)-SFO-(UA)-YVR.
As those who travel in Jpn would be familiar, the KIX layover involves going landside followed by security and exit immigration back to airside. However, ANA checks the bag at CTS through to YVR.

Because I will be in Tokyo earlier in the trip and do not wish to haul all my bags to Hokkaido, I would like to ship them to KIX via luggage-forwarding service and then retrieve it at their landside counter during my transit, bag-drop them (would be in addition to the one I check in at CTS).

I don't see why UA wouldn't permit that. We would not be exceeding the checked bag limit. But I wanted to double-check with them anyways. Not surprisingly, the phone agent was not helpful. I got a response along the lines of it being up to the gate agent.

I've done USA to Jpn transiting in NRT and checking in an additional bag with ANA on international to domestic layover. This seems like an almost similar situation, except it's dometic to international and it's UA instead of NH. Again I don't see why they wouldn't allow it. But I wanted to get some opinion on whether this would be a slam-dunk no-problem, or potentially problematic.