Would that be possible with the original roadblocks in place from the night before? Especially if they were looking out for him and his vehicle?
As I understand it, the roadblocks on the Monday night were just in Machynlleth - the A417 was not closed until the Tuesday morning.
My guess is that the police closed it because they had only a vague idea of where MB had recently moved - something like "somewhere in the Corris valley" - so they sealed off the north end at Dollgelau and the south end close to the Dyfi bridge. Dafydd-Powis is a very small police force, and I doubt that they had the personnel to block all the side roads off the 12-mile stretch that they closed. This may, in fact, be how MB was able to get his car to the garage without being stopped, by following the back route to Machynlleth that one of the locals described earlier in this forum.