"Private" struct members in C with const

This is almost certainly undefined behavior.

Writing/modifying an object declared as const is prohibited and doing so results in UB. Furthermore, the approach you take re-declares struct 2DPoint as two technically different types, which is also not permitted.

Note that this (as undefined behavior in general) does not mean that it "certainly won't work" or "it must crash". In fact, I find it quite logical that it works, because if one reads the source intelligently, he may easily find out what the purpose of it is and why it migh be regarded as correct. However, the compiler is not intelligent - at best, it's a finite automaton which has no knowledge about what the code is supposed to do; it only obeys (more or less) to the syntactical and semantical rules of the grammar.

This violates C 2011 6.2.7 1.

6.2.7 1 requires that two definitions of the same structure in different translation units have compatible type. It is not permitted to have const in one and not the other.

In one module, you may have a reference to one of these objects, and the members appear to be const to the compiler. When the compiler writes calls to functions in other modules, it may hold values from the const members in registers or other cache or in partially or fully evaluated expressions from later in the source code than the function call. Then, when the function modifies the member and returns, the original module will not have the changed value. Worse, it may use some combination of the changed value and the old value.

This is highly improper programming.