That leaves only UDF - would that be a viable option for flash media, in RW mode?

I've used UDF on external drives. Its only limitation was that Windows XP wouldn't write to UDF filesystems (from memory, Windows Vista onwards support writing to UDF).

It has been a nice little filesystem to use on thumbdrives where cross-platform compatibility is key.

The only downside is the sparse documentation on creating UDF filesystems on block devices.

The rw mode is quite neglected in UDF implementations, especially formatting tools.

Windows also assumes it's a disc, even though it does let you write if it's not XP.

You are begging the question