Problem is that the VIC don't see the ROM using the bus handler so 6502 need to copy data from ROM to RAM and that is why the slow 16 fps.
Even in that mode you can employ tricks to speed things up; Instead of placing data in ROM, the MCU could autogenerate a C64 program that writes data directly to RAM. This would of course put more load on the MCU.
As far as I know ultimax mode uses the vic handler and that handler is running in an infinite loop and I don't know if code could be injected to it.
Kim also wrote a comment on that handler saying it is using 100% CPU so don't know if that could be used.
I wrote it uses 100% CPU because it spends all the time in the interrupt handler, never allowing anything else to run, but that doesn't mean that you couldn't add more code to the interrupt handler. However, it may be tricky to write game code that meets the strict timing requirements of the interrupt handler.
If you don't need to handle both CPU and VIC-II reads at the same time, I guess that you could develop a new handler for VIC-II only reads and then switch between the two.