Mix multichannel and mono to stereo with JACK output

I use VLC with JACK but one issue I ran into is playing multichannel or mono audio on a stereo system.

VLC's JACK output just connects each audio channel to a channel on a sink and disregards mixing of center channels and LFE channels. This results in the center channel only coming from the left and LFE from the right. Mono audio only comes out of the left channel.

I know this may be what pro-audio users want from VLC but I would like an option to mix multichannel and mono audio to stereo and connect to JACK with just two channels.

I'm have experience programming in C so if anyone can point me in the right direction, I can probably implement such a feature and contribute it.