Alright, I've tried with the multi-tool. The standard offset plunge blade has a plunge depth of 50mm, and in a soft common brick I was able to plunge to that depth quite quickly, withdrawing a couple of times to clear the dust. Then tried in cement mortar, which was very easy to cut but hard on the blade. I think I could have cut all round a brick to remove it, if necessary.
Then tried a new blade on a hard facing brick. Rather difficult to get started, because of the vibration it kept jumping about on the brick. Once I'd got started I could cut into the brick, but very slowly. Finally I tried a paving slab, not a hydraulically pressed one. That totally destroyed the blade with hardly a mark on the slab.
All in all, I think the test proves this tool is not much use for chasing, although it'd be fine for a neat chase in plaster. This was an Aldi cheapo fitted with a standard blade though, and perhaps Bosch or Fein have a special blade for brick. I'll ask if they're at Toolfair in November.