It seemed the CPU power in an RB4011 or better RB5009 with USB would be good for say a WebSDR.
It very well may have enough CPU power for a lot of things
. And "native" docker on PC does support USB via --device to redirect USB...
I have an Ettus B210 that I use a SpecA, etc in a VM on Mac which works through USB redirection (since not a lot of direct SDR support), so the approach can work.
I guess you could see if your SDR shows up as a serial port in RouterOS (System>Ports in winbox, /port/print in CLI). Mikrotik does allow TCP redirection of serial data, IF there was a port found for the device – but not sure that help with I/Q data from an SDR & you'd still need something in the container to get the TCP network stream to the SDR software... But if you have SDR and RB5009, see if RouterOS detects it be step one (very unlikely however). It may be worth it to collect a supout.rif if you do this...and open a feature request at help.mikrotik.com for "USB redirection in container" (the supout.rif would have the USB device id for them at least).