Sound proofing a panned floor joist return run (basement apartment)

I’m building a basement rental unit in our home. We’ll be living upstairs so I’m doing my best to sound separate the upper and lower units. I’m using a seperate hvac system for the basement (baseboard heating) so we won’t be sharing supply and return lines. Also, I’m using Rockwool Safe’n’Sound in the basement ceiling in combination with resilient channel to deaden the noise coming from above.

My current issue is that the HVAC design for upstair, as it currently stands, uses panned floor joists for a number of return branch lines. This will leave a number of large sound proofing breaches in the ceiling, since the return cavities will be missing the rockwool insulation.

Is there a way I can fix this breach while maintaining panned return? My initial thought was to install some sort of insulation in the interior return, but I’m not sure what’s safe/to code in that case.

I’ve also wondered if I could install the panning higher up in the cavity and then just add rockswool in the space underneath.

I’m in South Western, Ontario, and the build will need to pass inspection.

Thanks for your input!