So a disclaimer, I have never read the C standard in full and I don't consider myself an expert, but this one about bit fields is one of many anecdotes I've accumulated over the years about portability. The rule is never use a bit field in a serialized file format because different compilers could interpret them differently. As long as a bit field never leaves the memory of the host that wrote it, they're fine. But in this case, you are expecting the bit field to overlay in a specific manner with a union, so I think you run into the same problem.

Making a library more portable in C usually means making the code an ugly mess of macros. So, first decide how much time and code clarity you want to sacrifice vs. the systems you want to be able to run it on. I have a large scale personal project data encoding library that I never finished in part because I tangled myself up in trying to make it 100% defined behavior C code. The world only has 3 major compilers: gcc, clang, and visual studio. If your code works on all 3, and its a hobby project, that's probably good enough. There are also vanishingly few big-endian systems left, aside from some embedded ARM processors. Also, declaring :62 will be a compile error on a 32 bit system, so that's maybe your biggest portability concern? But again, 32 bit is either old or embedded, so maybe not worth worrying about.

I believe that to make this portable to big-endian or 32 bit, you would need to make a macro to access the pointer and flags. Then you need to detect the endianness in preprocessor directives and set the macro to either a shift or a mask.