Back in the day I experimented with a peer-to-peer joystick port network on my 800, I never got it to be really satisfactory- but seeing the still-active Atari mod/demo community got me thinking about it again. I still have my 800XL and homebrew SIO cable, I have a few concepts related to writing a 'N:' device handler that would permit multiple Atari systems to exchange multiplexed data units via the usual IOCB subsystem; no usr() calls- everything through the usual I/O functions.
The idea is to use SIO2PC cables connected to a linux box that is only smart enough to be a sort-of switch eg; it knows ttyUSB0 is station 1, ttyUSB1 is station 2 and so on, such that a block addressed for 2 is queued for output on ttyUSB1. There are some compromises; some queueing is needed since the Atari systems will have to poll but I think the concept would be sufficient to do things like burst I/O between two systems, or use DOS to copy a disk file over the network to another station which receives it and writes to its own disk and so on. The network is more like a message broker system, so reliability and in-order delivery are guaranteed by the datalink so as to keep the device handler as small as possible.
Clearly its desirable to have a disk emulator built into the linux end, which seems like its turning that box into a file server and I/O hub. A little Pi would be perfect..
I'm aware of the Atari ethernet and the Multidash/Unconventional 2000 approaches, which are cool- my itch is to make a solution that works on top of standard SIO protocols and SIO2PC using the simplest possible server end.
I've always wanted to do something with the interrupt/proceed lines on the SIO port but sadly my DIY SIO2PC cable doesn't connect those pins- neither do the standard ones IIRC. Getting back to the Atari is a nice change of pace from the brutal complexity of modern software.