So... Replaced the FILE I/O with TCP-Sockets. As MAME can only connect to other servers, I've written a small "MODEL-1 COMMUNICATION BD" server app that manages connections.
Using Sockets, my Q6600 can handle 4 VR units (incl. Virt Mc Polygon).
Netplay may be possible. The original Model-1 has a max Bandwidth of 6 MBit/s (TOTX173/TORX173).
One Board sends about 20 KByte/s.
So bandwidth CLIENTSIDE is like... 2 Boards - 20 KByte/s inbound and 20 KByte/s outbound. 3 Boards - 40 KByte/s inbound and 20 KByte/s outbound. 4 Boards - 60 KByte/s inbound and 20 KByte/s outbound. etc.