After a bit of further thought and advice received, im coming to like another hardware option for the 'local' end of this project,
There is another Arduino, the Pro Mini, that is even smaller than the Nano, and, if the analogue pins are used as DIO, gives 20 I/O pins. Now, I need, if I also have the LOCK detect and AGC inputs, 30 I/O lines!
But heres the win - the Texas Instruments PCF8575 I2C 16-bit bidirectional port expander! 2 (or maybe 3, if the interupt is used) connections from the Mini, expands to 16 I/O lines. So, with 4(5) pins of the mini used for serial comms (RS232 and I2C), that leaves 15(16) plus 16 = 31(32)!
More than enough, and some spare!
Another two big advantages - both devices are 3v capable, and very small!
A disadvantage is that the Mini requires an off-board serial controller to connect it to the PC! But once programmed and working that is redundant anyway so isnt needed in the final working unit, its space being used for an RS232 driver instead!
Update - I think the Mini will actually only give me 16 available lines, plus 2 for Serial Comms. Might be a bit tight with just one expander! Perhaps only one or two spare pins!
A bit of work on the block diagram and it looks like the combination of a Pro Mini and a PCF8575 is the way to go! An RS232C SMD driver module to complete the set-up. The MOSFET needed to switch the Switches Vcc line can be a small single channel logic level shifter modified, or even a 2N7000 dead-bug connected to the Expander board. The AGC line for an S-meter, and the 2kHz LOCK tone, will input on a couple of Analogue pins.