I ran out of serial consoles and got an arbitrary serial<->usb dongle (two, 
actually). They attach fine:

uplcom0 at uhub1 port 1
uplcom0: vendor 0x067b product 0x2303, rev 1.10/2.02, addr 3
ucom0 at uplcom0
uplcom1 at uhub1 port 2
uplcom1: vendor 0x067b product 0x2303, rev 1.10/2.02, addr 4
ucom1 at uplcom1

But I am not able to send data through them - receiving and sending a break
works fine.

Anyone seen this? Sounds like a software problem in our driver.

Martin