Two things:
1) Another twin under 2000kg is the GA7 Cougar, but in my experience its performance is worse than the BE76.
2)
QUOTE
What do they do to register it for 1999kg? Do they actually do anything, or is it a paperwork exercise? If a paperwork exercise then surely the W&B will remain the W&B designed for the aircraft by Piper (i.e. it'll behave the same as a Seneca II in the USA).....just you might be overweight on paper?.....if you see what I mean
UNQUOTE
Yes and no. If the aircraft is built for a weight over 2000kg then of course it can physically carry it. But you cannot prove this capability on paper as the C of G envelope will only go up to 1999kg MTOW. If you load it over that and anything happens, you invalidate the insurance. You decide on whether the risk is worth it.
You also need to decide how often you will fly airways with it compared with how often you will need the MTOW to be over 1999kg. If one is greater than the other then you need to balance the relative costs.
The 'two up, full fuel' scenario is a feature of the aircraft and is not simply a paperwork exercise. It is to do with balance, not weight. Too much weight on one end of the seesaw makes it an unworkable game!