[BScout]

There are ways to do what you want but I can’t think of any that will be as cheap as another HDX plus interface.

Euphonix and SSL used to make MADI routers. With MADI you can do “substitutions” for every 8 channels which allows you to inject other audio into the MADI stream. In your case, you need to replace the 4th and 8th group of 8. If you aren’t used to Euphonix System 5 hardware, that would be a hard one to maintain. The SSL MADI X8 is a lot easier to understand (But you still need to convert your AES to MADI first with a stand-alone converter)

RME still makes MADI routers and AES/MADI converters where you can do things like this.

But at some point you will cross $4500 with these solutions and then it’s just cheaper to go with HDX + interface.