Hello Joerg,
the AMEL 55 mizzen mast is not designed to bear a mizzen staysail:
-the front halyard is going out of the mast with a fixed sheave. If you use it as a mizzen staysail halyard, it will chafe on the sheave's edges. You should try to rig an additional halyard, on a swivel pulley, attached with a textile loop to the triatic swage.
-the mast is not fitted with backstays. You will absolutely need to install one running backstay (windward side) when you set the mizzen staysail.
-81 m² seems big (it is 50m² on the AMEL 54) and you will need to drop the sail above 10 knots wind (true)
That said, I agree that a mizzen staysail is a great plus in light winds (beam reach/broad reach), and I'm sure many SM and AMEL 54 owners will agree.
If you are still in La Rochelle next week, I will pay you a visit.
Olivier.