Tate, I think you're taking the "all" part a little too far. (Edit - cyril beat me to the punch in a lot fewer words...)
V7.2 references the O5.4 Roofless Factory Hex rules for the uppermost location. V7.2 does not turn the upper location into a Factory Hex or Location, it deals with the missing roof. Just because the O.54 rules only existed for factories in RB, does not mean that roofless locations in VotG become Factories.
Factories in VotG use the O5 rules in their entirety (V6.1) in addition to the gutted rules applied from V7.2 (which references O5.4) for gutted VotG factories. (Kinda redundant, but the artwork is different.)
O5.4 - Definition and example.
O5.41 - Factory debris rules. Deals with TEM and LOS within the Factory. Contradicted by the V7.3 rules for TEM/LOS in gutted non-factory buildings.
O5.42 - MP/MF costs, nicely reinforced by the V7.4 additional cost to enter a non-factory building location in a gutted building. Yet still not Factory costs for entry. (I missed that before, conscripts moving without a leader into gutted buildings = no assault move; and advance = CX...)
O5.43 - LOS - I don't think this is going to happen much in VotG, but could happen from some of the level 2 locations into nearby roofless locations.
O5.44 - EC and weather - so EC and Weather affect the gutted building (no windows would tend to do that). Same thing for Wind and SMOKE in O5.441.
O5.45 - Indirect Fire - Allows indirect fire, without regard to size of the gun or SW. This is repeated in 7.7 for some reason, but 7.7 affirms the +1TEM per level above rule that would be missing if O5.45 is the sole source for TEM in gutted buildings.
O5.46 - Rubble - still possible.
O5.47 - Sighting and Aerial TEM - repeated in V7.6 with additions that affirm the normal rules for the non-roofless locations.
O5.48 - NVR and Illumination - I'm not sure this needs any chapter V references, since I don't recall any interior gutted building locations on the VotG map. (And I seem to recall that NVR stops at the first building location, and starshells can be fired from a building location as long as it's not internal.)
Generally speaking the O5.4 rules are amended in V7.x when Factory behavior would be the norm, such as interior TEM and LOS or MF/MP costs. On this basis alone, and since neither V7 nor O5.4 reference any extra capacity for Gun setup (just indirect fire), I don't think the upper-most level of a gutted VotG building can contain a Gun that would not otherwise be able to setup there.
Being able to fire indirectly from the uppermost location, requires that the indirect fire weapon be allowed to setup there. We've also covered the "loophole" that allows an 81/82mm MTR to be carried to that location as a DM SW counter. This might not be so clear if there were larger mortars available to either side, such as a 120mm that can't DM.