I hate to be *that guy* but the Propeller Hat (Pimoroni, currently out of production? I have one, however, so they weren't last year, at least...thepihut.com has them...)https://thepihut.com/products/pimoroni-propeller-hat
...is NOT the same board as discussed above and one doesn't need fancy FTDI "up and down the USB stack" to merely pass a serial port.
The Propeller Hat is simply programmed by UART serial directly via pins on the Pi's 20x2 pin header.
While one would still need to develop the Propeller application in Raspbian, one could store these on SD card and then likely load them from an Ultibo application, via the standard UART unit? https://ultibo.org/wiki/Unit_UART
I'm not entirely clear on loading files from an SD card in Ultibo, but programming the Propeller with pre-built apps seems straightforward, at least.
I'll eventually report back on this, as this is precisely what I will need to do at some point in the not-so-distant future...