Is it OK that I accidentally swapped TX and RX on a UART bus?
It's quite unlikely that there'll have been any permanent damage.  Typical LVCMOS output stages can survive contention with each other - they will source/sink 20 mA or so, which is usually below their damage limit.  But the details do depend on the ICs involved, you could check the datasheets if you're thinking of building these particular parts into a high reliability system and are concerned about latent failures.