@lleachii is correct, there is nothing special within the kernel about this address range, it's treated like every address range. To be sure I just created a dummy device and gave it 240.0.0.1/4 as an address and then pinged it... it worked fine on my linux desktop machine. At this point I think you need to just get yourself a device and install OpenWRT on it and start playing with it. Just give one of your interfaces a 240.0.0.1 or similar address in the configs and see what happens. I don't anticipate any problems when all devices are under your control.