Again, I'm not an expert, and I might be wrong about this, but I think you are approaching this in an overly difficult way. That's not a criticism (I seem to approach everything in my life from a difficult angle too), but there are numerous applications that should allow for very easy L:Var manipulation. I'm sure FSUIPC can do it too, but I'm not sure that it requires register manipulation.
The B:Var is the new MSFS interaction system, which can be very useful, but it's also incredible abstract and convoluted for developers to set up, so it tends to be underutilized. Not all 3rd part applications have access to B:Vars, so I would try to avoid them wherever possible.
