SL30 had this feature 30 years ago. That software upgrade never came.Given that this and the Trig can see two VORs and the radials at the same time, seems to me they’re both just a software upgrade away from having at least a rudimentary RNAV capability, although DME would definitely enhance accuracy.
I need to re-read the SL30's serial port protocol. Now you have me wondering whether it can be done with an external box.