Try it. You'd be surprised how easy it is to do (I ride without risers and in relatively forward position, so there is not much crouching to do to reach the knob).I don't think I could adjust the pre-load on the fly with the ST13. Knob turns too hard for this wimp to do while on the saddle running down the road.
When I bought my 03 second hand, the knob was very hard to turn because the adjustment piston on top of the shock absorber would stick. From a post on this forum, I found out how to lubricate the piston and the knob is now much easier to turn. No problem doing it on the fly. I have it pretty tight to begin with, and all it takes is a turn or two up or down to make quite a bit of difference.
Looking at the convenient location of the knob (+ on left side for left hand), I always thought the intent was to be able to adjust on the fly. Works for me. Strange enough (or is it by design?), it is actually easier to adjust while on the saddle than with the bike on the stand. The reach and wrist leverage is better.