just thinking it would be something different, like the .50 conversion of the ROA, or the .45 brimstone walker...
Yes, well the 50 ROA has the benefit (for what it's worth) of using the same round ball as your rifle, quite different from having a 45 rifle and a 44 revolver which take very different size balls. I haven't seen the term "45 Brimstone" before, but the 45 BPM (Black Powder Magnum) has the benefit of making use of the Walker's extra cylinder length, allowing one to use Walker power lever charges in a metal cartridge. So in each instance there is a rationale, or a goal, which says "I want to be able to do x, and there's currently no way of doing it, therefore I will do a custom build to accomplish x.
I think you have a potentially nifty idea. I just haven't heard what "x" is yet. If we knew "x" we'd be able to talk more clearly about this.
Also I believe the percussion cylinders for the 36 and 44 Remington repros are the same exact exterior dimensions in the same frame. Conversion cylinders may very well be another matter.
None the less, there are six shot, 45 Colt conversion cylinders for the Remingtons unless I'm very much mistaken. In that case, you see that there is in fact room for a six shot 36 SM (36 Suzuki Magnum) based on a 45 Colt case. You'd have to make your own cylinder from stratch though, most likely. The six shot 45 Colts use an angled chamber to make room for the case heads.
Otherwise base it on a 38 Spec cylinder and machine it out to take a 44 Colt case and a .375" larget bullet. Again; the 44 Colt and 44 Remington cartridges were originally created to fit in the converted percussion cylinders, so if nothing else, there's your answer-- You'd just have to start with a 38 Spec. cylinder and open it up accordingly.
But first things first; what is "x"? Maybe there are multiple ways to accomplish "x".
It's totally doable in some similar form, one way or another.