If you're stuck with a Belkin WeMo where you had unfortunately upgraded the firmware, alas, the only way I've found to get OpenWRT running on it is to break out the soldering iron. Unless another vulnerability is found... If you're running firmware version WeMo_WW_2.00.2176.PVT or lower, check out The Easy Way.
Vulnerabilities aside, the firmware (even the latest version) on the Belkin WeMo is just plain flaky. I've had multiple issues where the device appears to just "hang", flashing an orange error light if it temporarily loses its wireless connection (which seems to be around once a week). Fed up, I decided it was time to grant them a new lease of life, by reflashing them with OpenWRT.