Despite only containing constant values, the following causes a C++ compiler to allocate the struct in RAM and generate a static initialiser function:

const struct {
    mp_obj_t y = MP_ROM_INT(42);
} x;

Forcing the compiler to treat it as a constant expression explains what the problem is:

constexpr const struct {
    mp_obj_t y = MP_ROM_INT(42);
} x;

in constexpr expansion of ‘<unnamed struct>()’
error: value ‘85’ of type ‘mp_obj_t {aka void*}’ is not a constant expression

C++ forbids the use of pointers in constant expressions unless they refer to a static object, a function or are nullptr. There's no way around this.

Object representations A, B and C use a void* type for mp_obj_t. This makes it impossible to initialise an mp_obj_t value at compile time for them in C++ code. Changing mp_obj_t to uintptr_t makes this work but then mp_const_obj_t is no longer const in any representation. Perhaps the type could be changed to uintptr_t only if __cplusplus is defined?