I have experience with the blown cellulose in existing non-insulated walls. I have never used the foam system. A hole would need to be drilled into each stud bay (approx 1-1/4″) to insert a tube that would fill the bay with cellulose. I assume that you have wood studs behind the plaster. The downside to this application is that you can’t see what you are doing. Many times there are wood blocks or conduit or some other obstruction within the wall that blocks the movement of the insulation. Also in your case you would need to insulate from the interior and drill and patch the plaster as it would be quite an ordeal to drill the brick and attempt to patch that. I assume the foam would work in the same manner and I would think it would flow better around any obstacles. Even if you are left with some voids you are still better off than the zero insulation you have now.

Again, I’ll bat this around with Geoff in a couple of weeks so keep listening and thanks for the question!