@luz I too suspected that the modem control signals not being connected was what makes the UARTs "lite". Unfortunately in my application the UART on the other side doesn't support automatic XON/XOFF – just automatic RTS/CTS – but if yours does that might work.
@ccs-hello the problem with "the sender should just slow down" is that the receiver is variably slow. Most of the time it can saturate the connection at 115200 baud, but if one of its other subsystems causes a load spike it can fall behind. Since RTS/CTS handshaking forces the sender to wait when the receiver's FIFO is full I can push the limits on transfer rate without having to worry about data corruption if the receiver takes too long to pump the receive queue occasionally. Dropping the baud rate to something the receiver can handle 100% of the time instead of just 95% will cause something like an 80% slowdown in overall system performance, but if I can't get RTS/CTS flow control to work properly I guess I'll have to take it.
Once I've got the prototype for this project put together I'll see if I can get RTS/CTS working thorough the Omega's GPIO by setting an interrupt and disabling transmit in software. I'll probably have to adjust the automatic RTS threshold on the receiving UART, but I can do that and it might just work. That'll be a while, though.