I've set up USBDM 22.214.171.124 with Kinetis Design Studio 2, running on my Ubuntu 14.04 64-bit box. It's driving a Freedom KL26Z board modified in the expected manner to act as a SWD programmer for an external device - and that external device is a standalone MKL26Z256M4. The KL26Z has the USBDM OpenSDA v1 firmware. Programming and debugging from KDS work fine.
However semihosting does not. There aren't any explicit options for it in the USBDM setup, and typing 'mon arm semihosting enable' into the GDB console does nothing. The target device stops on a breakpoint instruction inside initialise_monitor_handles() (which is the expected system call used for semihosting).
I haven't been able to track down any specific information on semihosting and USBDM, so I suspect it isn't currently supported (or indeed requested).