I sure hope E* is testing MPEG4. In fact, they said they are testing it, and so far it isn't passing the mustard. So I wouldn't hold out much hope for picking up meaningful content from 148 in MPEG4 in the near term.
Anyway, it is highly unlikely that E* is going to offer unique, new channels in MPEG4 from 148. You test out on 148 and when it is ready, and you have a coordinated program to replace 8PSK receivers with MPEG4 receivers, then you go into production on the main birds. As there are plenty of existing HD channels to use for testing, there is no need to use new channels for testing.
My hope is that E* plans to put as much programming as possible on 110, 119, and 129, so that a D1000 is all that most of us need.