I believe it’s not possible because the GPU doesn’t dynamically figure out whether it’s a pointer to device or constant memory.
You can see this by looking at the PTX output code. Pointers to constant memory are statically dereferenced with a ld.const.type instruction where loads from the device memory have the ld.global.type instruction.
You can, however, make a conditional memory load without letting a pointer conditionally point to different memory types:
// conditionally dereference either the constant or the device pointer:
T p = (condition) ? c_Array[i] : d_Array[i];
Edit: Changed pointer indirection to array subscripts in code example.