Recording HDTV to HDTV
 

Is it possible to use tmpgenc to compress hi-def mpg files, keeping the 1280x720 resolution? Of course, the resulting files would not play on dvd, but from mce hard drive.

Don't use MPEG2, go to h264 for that.
Beside this, yes it is possible with tmpgenc (contrary to CCE that is limited in output resolution).

Note: I think that HD ts are 1:1, not 16:9 nor 4:3. Be aware to set correctly the output parameters.