I’ve been mulling that idea around as well. I’ve got two Presonus 1824c units collecting dust in my rack since I got the QU-16. The QU-16 is not the audio interface I thought it would be. It’s fine as a mixer, but it’s too complex for my needs at home (mostly synths). I used to use channel inserts as individual out from my Mackie ProFX16v3 feeding the 1824c units that were chained together, and then also used some sub groups, but it still didn’t quite get me to 16 individual outs.. Close, but no kielbasa. I don’t know how to do what your asking but hopefully someone will chime in.
Maybe you could use the Stereo ALT OUT, it’s 1/4 inch balanced and can be routed to any output mix.
To use the stereo pair mix outs you would need to bring up the channel send levels on those mixes. The mix sends can be set PRE or POST fade with various channel pick off points depending what works best for your needs.
I think all and all you should be able to output 12 channels. If you link 2 channels and pan them hard LR, do the processing in the channel strip and pick off post fader it should work. Even for the mains.