Modulation ideas for old and long analog bus cabling
WIth powerline modems it is possible to get 500Mbps, 300 meters in point to point but this requires equalization training for each point to a hub. 

In P2P this would have to be trained for every combination of P2P and saved in the baseband modem which has to be done in parallel. So if anyone can be talking then you must have a low speed negotiating phase when idle.

300KHz with Er=4 has a full wave of 500m so using 10% of wavelength for  transmission line low reflection bandwidth leaves 30kHz of low reflection bandwidth at 110 Ohms =/-20% est. at HF limits which could support 250kbd easily and possibly 500k with EQ using ODFM. 

But PSK,bi-ph has the best resilience (Steepest curve) for BER at  low SNR.  Compression factor depends on content if already compressed or not. FEC is desireable.

Consider R&amp;D budget cost vs COTS modems,  I would be looking for  HD-PLC with Ethernet to a hub and loop back to get any to any using point to point PLC ( power line carrier) technology but without the AC power using choke grounds and RF caps to suppress AM ingress.  

In other words a bus/tree  with MIMO Hub loopback to get P2P at max rate.

For low bit rates (TBD) simpler solutions exist...but  assumption was up to 256kbd.