hey! sounds like an interesting idea
the problem I see with that is the limitation of jack1 : if you can set up jack to use this alsa device, you won’t be able to use any other device. Jack2 shall be able to handle more than one device at a time (IIRC). But maybe some asoundrc wizardry can solve this by defining a single virtual device that binds your different h/w devices. But I don’t know how good jack handles made up virtual devices. It is probably preferable to have jackd to talk directly to hw:x … but if you get anywhere, please report back.
there’s a general limitation with having more than one sound device which is that clocks can drift apart. The slightest mismatch in samle rate between sound cards will ultimately lead to a noticeable time lag. It will never work unless devices run on the same quarz or are otherwise hardware-syncronised.
For a bluetooth monitor it would maybe be OK to drop a sample every so often or slip another one in, but jack doesn’t do dirty tricks like that.