smazzer your particular concern, as described by others above, is not relevant in the case of the sync buck...but yes, your point is very relevant in cases such as the phase-shift-full-bridge, where your concerns can result in failure of the fets....because often there is not enough reverse bias across the (internal and/or external) diode when the fet is on...then the top fet turns on and their is severe reverse current in the intrinsic (or external) diode...sometimes resulting in failure......................................your point is also a concern in LLC converters when they operate at lower f(sw) than the lower resonant peak.
But yes, you are well cognisant of the fact that in order for a fast diode to turn off properly, it must undergo reverse recovery.....those charges must be swept out, as you imply.
Of course, with a low voltage sync buck, many would say just use an external schottky, and its problem solved because they dont have reverse recovery