Thanks for the replies, everyone. I just figured out I can do this, does it make sense to do it this way, seems way easy.

Use the adjust qty/value on hand and simply enter all of my inventory as of 12/31/20 and it's value, which would create it's avg cost on 1/1/21 and the adjustment account would be Owners Equity. Everything seems to be checking out perfectly as far as I can tell.