Agreed a serious "real world" problem. having said that people are working on it. I have a search around earlier and found there is a draft RFC for the UDP encapsulation of SCTP (http://tools.ietf.org/html/draft-ietf-tsvwg-sctp-udp-encaps-...), this combined with a soul destroying use of a zero data payload keep alive to fend off NAT stupidity, and maybe a server side end point abuse of port 53 to keep "Carrier Grade" NAT quiet might be the trick. All this should work on mobile platforms.
In general doing this "properly" is an exercise in icky compromise.