It sounds to me like the stack space required by TailCallHelper is proportional to the largest amount of stack space ever consumed by a single call of the tail

It sounds to me like the stack space required by TailCallHelper is proportional to the largest amount of stack space ever consumed by a single call of the tail recursive function(s) it's assisting, whereas without it, the stack space required would be proportional to the depth of recursive nesting. So using TailCallHelper you should be able to guarantee arbitrarily deep tail recursion.