This was added to the Linux kernel some time ago. In fact, a majority of the discussions took place in these forums by the person originally responsible for its introduction.
Despite this, since the RFCs note that the Class E IP space is for a new IP technology (i.e. a technology other than Unicast, Broadcast, Multicast, etc.), its use for such networking purposes as you describe would still be invalid.
Additionally, be handled as invalid or unrecognized on other OSes and devices - and hence may result in undesired behavior when used within your network.
Here's the original thread: Can OpenWrt Route 240.0.0.0/4?