Amazing work on the linkups. Wingwar and Virtual Formula look great.
I had a go at using shared memory and two instances of Mame running on the same PC to get two virtual GPRider pcbs hooked up together a while back. It uses a dual port sram chip between two System X boards to handle the comms.
Unfortunately my PC wasn't fast enough and the emulations lost sync quickly.
You might like to try it on your system.
The shared memory is at 0x2f0000-0x2f3fff. All I did was add a memory handler that mapped the memory to shared memory (look in the SMGP init for how to add that handler, its in the same place).
If I recall correctly, GPRider only actually used 32 bytes. (but it has been a while)