nice little drawing to show my setup:
https://docs.google.com/drawings/d/1ARIwD0tSf3jNeI8mrfqR9Him-yyEeX5LRlICsK0Bi9c/edit?usp=sharing
FWIW (not much) - I have 12 shades in total, 3 EZIO8's, and 3 serial PLM's to control them.
I had three approaches labeled (A) and (B) in the diagram. Each involve trying to get my "master modem" (the 2413u connected to OH) to talk to some component on the Insteon network to control the blinds.
Approach (A) was my preferred and is exactly as you describe it. Get the "master modem" to control and respond to each of the EZIO8 modems. Houselinc however does not recognize/support the EZIO8s. Short of another tool to help me do this, I'm stuck. I did reach out to insteon support to see if there was a way to use the "set" button, but did not get a helpful response.
Approach (B) was my backup. I have each blind connected to a 6btn controller in the house. My original plan was OH could signal the blinds through the scene buttons indirectly. Unfortunately, while OH can respond and control the scene buttons, it does not appear the message is relayed to the blinds.