Welcome to the world of unsolvable issues. You can't have both. There is no such thing as isolation and gain before feedback, when you have a 7 to 9 foot long wood box full of strings and resonator.
Your piano player is going to have to get out of the 1950s. Spend the 2k bucks and MIDI the silly thing. The piano player will not know. The sound module can be located under it.
The thing to remember is that the piano was not made to be direct mic'd. It is an unnatural sound. Then the style and technique of each player makes it sound like a completely different beast. A percussive over-th-strings-jazz set up was required by on of our players, because she was so soft. Then when the gospel banger got up there, a single Crown PZM on the resonator rocked.
These players have to understand that you have limited resources and one piano with many players and tones. In a perfect world they could have their own. The only way that they will be heard is if it is MIDI'd, otherwise, I am not putting it in the mix, and I am certainly not putting into the monitors. Gosh. Sorry, I had a flashback. Where are those green pills?