After full CPS transformation, you absolutely can allocate the activation frames on the stack. Cf. CHICKEN Scheme that does precisely that, and more generally,

After full CPS transformation, you absolutely can allocate the activation frames on the stack. Cf. CHICKEN Scheme that does precisely that, and more generally, uses the stack as the 0-generation. When the stack hits a certain depth, it longjmp's to the GC, copies whatever is alive to the heap and restarts the current continuation on the now-trimmed stack.