MiSTer is FPGA platform primarily. So it's not targeted as simultaneous use of Linux and FPGA.
When Menu core is executing, basically nothing happening on FPGA side and HPS doesn't have to monitor the FPGA events. That's why switching to HPS/Linux while in Menu core is harmless and doesn't affect neither HPS nor FPGA parts.
When other cores are executing, HPS is busy by monitoring the FPGA events to supply required data timely. One of CPU cores is busy at 100%, another is always ready to provide the data from the kernel. So, other HPS/Linux tasks will distract the CPU and you may have different kind of problems like unresponsive keyboard/gamepad, lost FDD/HDD data or other problems.
So, there is no plans to provide dual HPS+FPGA functionality to the user except some cases specifically developed and tested for such use.
From the whole project the only function is interesting is how they forward networking through specific driver. So it may lead to a faster networking for Minimig. Otherwise the functionality is unrelated to Amiga.
If you want to listen MP3 on MiSTer, you don't need to use Minimig. Use Menu core and switch to Linux and then listen MP3 there.