Sometimes unorthodox approaches are not used, because people are stuck in the (imperfect) tradition, but sometimes that may not work. Are you doing it to save the material or to save 13 cm (12 cm Spanish brick + 1 cm bell/wall gap) of living room space? I want it to work for you and it would be sad if you had to disassemble the heater in a year or two to build the continuous bell. Of course we will not know until you try and operate it for a few years. I can imagine what kind of massive heart attack it would give to authorities here who already do not tolerate even single skin :) Your design can be called "half-skin". Please also consider that this region of Spain has some seismicity. Good masonry relies on gravity - so buildings and structures are designed to work even without the mortar, by interlocking pieces, thick walls for stability, erecting thick buttresses for vaults and arches, cornices with proper cantilever per brick, etc.
Have you checked if the floor will support the heater weight?
Permies is a forum where people usually do not boldly say "do not do it" assuming that there is some unexplored path that may be discovered. What I can only say that in hundreds of heaters and builds that I watched/studied all of them had a normal bell - similar like I never saw an engine with half an engine block - the other half being car body.
Despite the fact that I would never do it I remain curious of the outcome.