HI again, lextoo.
Of course, what I should have added was that I was only answering your question as put - "how much support can you get from a grouted and reinforced wall".
During construction of the wall you have an entirely different state of affairs. Until the grout and mortar has achieved substantial strength, your "wall" will be merely a loose assembly of blocks, with weight but little structural integrity. In that condition, you will get no buckling restraint worth considering.
If the lintel is to be supported until the wall has achieved structural capability, then that would not be a problem. If not, then the lintel will have to be capable of taking the full load without any assistance from the wall.