The KL2x series at least supports low-speed when in a host role.
(I think I even tested this.)

It doesn't seem to when in a device role, though.

If you need a low-speed device, you could use the SiLabs C8051F3xx
family, which can be full-speed and low-speed. And then there's
always V-USB :-)

 Thanks Werner! It seems to be a little nice micro to test it. I've some ideas for some kind of android peripheral, so this thing could be great with such a low power consumption as few uAmps :D

V-USB is always the last resource to use. I prefer to have as much flash memory as possible to code ;)