Weighing in late, but in 100% support of JasonTX.
Quoting CopeDaddy:
"The B-20 may also be a penalty enforcement spot for fouls by Team A during the kick play
To me this means the play during the kick. It does not specify before or after the kick ends. Blocking below the waist is not allowed during a kick play, not just during the kick, I see it the same way. This is what makes it special."
C-daddy - the rule says it: "...during the kick play..." By definition, a scrimmage kick 'play' is from snap until the ball comes into possession (or it - the kicked ball - is declared dead). Once the ball is in possession, the kick 'play' (but not the whole down) is over, and penalty enforcement obeys Basic Spot enforcement rules for fouls during running plays thereafter. So, fouls before the runback and during the runback are very different, as JasonTX pointed out.
As I once heard RR say, this is one time when a team can have their cake and eat it too. They can elect the illegal touch/bat which effectively yields a touchback as well as makes the end of the kick the B-20. So, PSK fouls are enforced related to the B-20, and qualifying A fouls can be added to the B-20 (as the "spot where the dead-ball belongs to B"). Live-ball fouls that occur after the end of the kick are enforced as any running play.
As with any illegal touching situation, offsetting fouls cancel B's illegal touching privilege. The down is repeated. The exception to that is, of course, the "clean hands" situation. B can decline offsetting fouls, decline A's penalty, and then A can elect to accept or decline B's penalty, which might be a PSK foul with the B-20 as the end of the kick due to A's illegal bat/touch in B's end zone.
Easy,eh?