I have not done it, however I have had to do something similar on the Shipping Screen, where Epicor’s Adapter had a MessageBox hard-coded and you could not suppress it.
I ended up handling the Adapter logic myself by using the BO (Business Object) instead of the Adapter and calling args.Cancel = true; which tells Epicor that you handled it. (But you want to Trace it first to make sure you call all Methods that Epicor calls in the same order (sometimes there are more))
I know this doesn’t solve your issue, but it may drive an idea.
args.Cancel = true; // stops Epicor from executing its Adapter, since I've handled the same logic myself so I can control the msgbox