Allowing a BJT to turn on and turn off at different set points
What about this: -



When the switcher starts, this is detected at R4 and feeds the gate of a MOSFET which then clamps the base of Q1. I've added a 1nF capacitor to ensure that ripple is reduced on the signal feeding the MOSFET's gate.

You may also consider not clamping the base completely and fit a resistor in series with the MOSFET's drain - maybe 470k.