This works, even with a 1 microsecond pulse 
Boards will be sent to Laen tonight.
Edit: Okay, it almost works. It can get in a stuck in a state where the SCR is half open and the capacitor stays at 1.25V. The drain of the BS250 stays at 0.68V in that case. Suggestions to fix this are welcome.
Edit2: Fixed with a resistor from the SCR gate to ground Boards will go to Laen today!