I am not so sure Wayne.
B9.4 is the only place I can find where a gap is discussed.
B9.4 The cost for Infantry/Cavalry to cross a wall/hedge hexside is one MF plus the COT of the hex moved into. The cost for various types of vehicles to cross a wall/hedge is indicated on the Chapter B Terrain Chart. A vehicle failing a Bog Check for crossing a wall/hedge is Bogged in the hex it attempted to leave. Wall/hedge hexsides are never expressed as a COT, but rather as an addition to COT. Therefore, when a unit must pay double its normal MF cost to cross a Crest Line it pays double the COT of the hex plus the one MF for its hexside. Some wall/hedge hexsides such as 6W9-X9 have obvious gaps in them which can be crossed without paying the wall/hedge MF penalty (by using the road if one exists).
The one Q&A I see states:
Q: May a wall/hedge hexside with an "obvious gap" (but no road depiction) be crossed without paying the extra movement cost? May walls/hedges with "obvious gaps" (whether there is a road or not) be crossed by expending the additional movement cost for a wall/hedge if desired?
UA: Yes to both. {4}
Unfortunately, it does not ask anything regarding bypass movement, however, it does not specify infantry or vehicles (so applies to both).
For VBM, D2.3 states in part "Walls/hedges are considered extensions of hexsides for this rule." but that is talking about the hexside along which the vehicle is bypassing (i.e., the width is measured from the inside edge of the wall). this is different than if there were woods along the hexside. If there were woods, the vehicle could still bypass the building, but would need to go through the woods.
So a vehicle can cross the S5/R4 hexside without crossing the wall. What limits the crossing to the vertex, or in this case just inside the wall depiction? Given per D2.3 any wall depiction is treated as an extension of the hexsides the wall is on. Does that mean the partial wall along the S5/R4 hexside is also considered an extension of the hexside and therefore immaterial to the question?
Do you believe infantry can use bypass for the same move? I believe they can, and I believe for the reasons stated a vehicle could do the same. I would agree that if there was any other type of obstacle it would be blocked (or at least you would have to pay to go through the other terrain), but I think it is clear that walls/hedges/bocage are treated differently for bypass movement and legality.
It would have been nice to have the A4.31 example include a variation that has a gap in the D4/D5 wall and tell us what the costs would be.