I dont suppose you could run the mics to your firepod and just give foh the line outs? Or get the direct outs from foh's board? I think it'd be better to split at line level, doesn't splitting an already weak mic signal just make it even weaker? Plus with phantom power that's tricky.
If I'm the sound guy, and you were on my house PA, I'd say no way. I'd find another way for you to get a signal, AFTER I've set the gains and have my signal, at a point where you can't mess with the signal going to my mics, pre's, rack, and PA. Especially if I thought your device was home made, I don't know if your device is electrically sound.. Maybe it'll send my mics 96v now, maybe it'll fry my board or pres, who knows.. You could stop a show or damage equipment I'M responsible for! There's easy ways to tap a signal securely, if all the busses and direct outs are used up, a simple patch bay can split the signal anywhere after the pre.