Center-tapped transformer and inductance calculation for transformer driver
The reason I'd like to know the inductance at the secondary is to
  calculate some RLC filter with it (to match the output filtering to my
  desire),


You are misunderstanding how this forward converter works; it appears you might be thinking it operates like a flyback converter, which it doesn’t.

The inductance that can be used in the secondary in terms of usefulness as a filter is leakage inductance and not the primary to secondary projected magnetization inductance. Leakage inductance is not given in the data sheet but, can be assumed to be between 2% and 5% of the magnetization inductance as projected via the turns ratio squared.