IPv6 jumbograms are a whole different kettle of fish, as they are really handled at the IP layer, and are supposed to only be for cases where you have an MTU > 65,575 bytes. As that link describes, the UDP protocol itself doesn't do any fragmentation or reassembly of those packets. RFC 2147 is really just an update to handle the fact that IPv6 can have a single packet that is larger than an IPv4 packet.
