I am thinking RS232 wire to a USB adapter (makes a change from the other way round) to 'feed' my PAW with GPS position information
This is possible, and essentially what I did.
..and, possibly, an RS232 wire to SMA adapter to 'feed' the same position to my Funke BFI57 'back up' instrument (I haven't seen if this adapter is a real thing yet). Any snags with this please feel free to shout out
SHOUT!! That's not going to work. The SMA connector isn't a serial input, it is an RF antenna input. The BFI has its own integral GPS receiver (pretty much like the SP12) and really needs its own dedicated GPS antenna. As it is your backup device, it would be best to reserve a spot for that antenna and not try to mix it with any other GPS RF side. You'd re-introduce a single point of failure, which means you risk losing the primary and backup at the same time.
Now, going a bit techie, does anyone know what baud rate would be acceptable to the PAW box, assuming it was being fed the GPS position as described above? The SP12 usually uses 115200 as the default and, I believe, this allows SDA 2, SIL 3. The baud rates of the SP12 can be changed and, obviously, this would not affect the PAW as it doesn't need those 'certified' numbers but I have no idea if going to a different baud rate reduces the SDA 2, SIL 3 of the SP12. I think I'm talking Greek at the moment 
As Lee already replied, PAW will work fine with 115200 Baud. Some other devices (notably RS-232 connected ELT's and transponders) require 4800 Baud or 9600 Baud. As the documentation of the SP12 shows, this will reduce the number of position messages per second. The simple reason is that at 4800 Baud you can only transmit approx 480 characters per second. If you add the length of all NMEA sentences ($GPGGA, $GPRMB etc) you simply can't fit two full sets of position data in 480 characters. At 19200 Baud you are already up to almost 2000 characters per second so you can have four full position sets in a second, enough to allow SIL=3. At 115200 there's plenty of room to transmit position from the SP12 to the attached devices several times per second.
So unless you must feed RS-232 at 4800 baud to some other device, leave the SP12 at default 115200. Better yet, if you have something else that wants 4800 or 9600 Baud, you might be able to feed that from your BFI57 which also supports NMEA out.