I can't answer your question, but we live in hope. I guess no MP developers who know what they are doing live where MHEG-5 is being used.
I have EPGCollector reading the MHEG-5 data here once a day which creates a TVGuide.xml file which the xmltv plugin reads. But the plugin doesn't read some useful information like program and movie classifications which is a hassle as we have kids. I've brought this up in the relevent forums a few times over the last 4 years but nothing has been done about it.
The plugin also chops off long program descriptions in mid sentence which is annoying.
Hmmm... This is not strictly true as the user mm1352000 lives in NZ and he is the lead MediaPortal developer for the TV side of things. He is really helpful and has helped me through the odd problem I've had with MediaPortal. I'd be really curious to know where he sees things heading with MHEG-5 and if it's perhaps something that is being looked at for MediaPortal 2.
On the collection side of things, do you have a DVB-S card in your HTPC? I have tuned all my Freeview channels on this card and merged them with the DVB-T channels so it uses the EPG from the satellite rather than the terrestrial signal. I have then made the DVB-T card a higher priority than the DVB-S so this gets used in preference. It works well, and I'm pretty sure I don't get the text truncated. Not sure about the classifications as I don't use them, but will check.