The current word, as far as I've been able to gather, is that a MEP Class Rating gained on the DA42 will be valid only on similar FADEC, diesel twins. Differences training and a further LST will be required to extend the privileges to a 'conventional' MEP aeroplane.
As to the IR - the only thing I've managed to get out of the FEs is that the aircraft must be capable of displaying 'limited panel' instruments, defined as magnetic compass and turn needle/co-ordinator. However, I have heard rumours of a proposal by the French DGAC to limit a JAA IR gained on a DA42 to aircraft fitted with similar integrated avionics (e.g. Garmin 1000).