how bout you d a m/s recording.
have the mics go to one channel and the sbd go to the other and do a M/S mix in post. just have your SBD be your Middle and your mics be your side. It is not the typical way of doing a MS recording but it might be neat. you need wavelab or soundforge to do this.
That's quite clever! He'd still need a figure-of-eight mic to do m-s though.
And it sounds like Joel really wants both aud and board so let him have it. The only trick is going to be setting levels with the 2 channels coming from different sources. I've met Joel and know that he owns some attenuator cables. You'll have to put those in the soundboard path and/or probably ask the sound dude to cut your levels down so you have something usable for both the board and mic signals.
Then in post, you can load it into an editor and play with the channel balance (CEP has a channel mixer where you can give each side whatever proportion of left/right source). The end product will be mono but you can choose the balance of board/mic.
As far as plugs/adapters, the mono adapter will likely draw from both left and right contact areas on a stereo plug.
That's gonna be one ugly setup!