We’ve had some discussion at the shop and the general feeling is that the issue remains controversial and it’s therefore necessary to institute a local ruling on the subject.
Here is what is proposed and remains open for discussion at the moment: In missions that use the Prepared Positions special rule (see page 263 and 264), obstacles may be placed in your own deployment area or no man’s land (page 262). Obstacles include: wire (either cat wire or low wire entanglement), mines, booby traps, and tank traps. They must be placed at the beginning of the game. Therefore any pioneer / engineer supply vehicles held in reserve would lose the benefit of placing obstacles. They cannot be placed such that teams would be in impassable terrain or have crossed impassable terrain to get where they are (Battlefront ruling).
However, as platoons that are held in ambush are considered to be placed on the board at the beginning of the game, mine and booby trap obstacles may be held in ambush and not revealed until the ambush is sprung. All other obstacles such as wire and tank traps must begin the game on the board. As only platoons can be held in ambush, supply vehicles held in ambush must be an integral part of or attached to a platoon at the time the player determines which platoons will be held in ambush prior to their deployment. As well, transport teams, in this case pioneer / engineer supply vehicles, that do not have passengers (page 46) must be attached to Combat or Weapons platoons in the same company (page 260).