You've described, basically, a custom type that is 8 Options<u8>s. If you start caring about performance you'll need to roll your own internal Option handling.
There's no "manually" about it. There's only one way to implement it in C, ie eight booleans and eight uint8_ts as I described. Going from there to the further optimization of adding a `:1` to every `bool` field is a simple optimization. Reimplementing `Option` and the bitpacking of the discriminants is much more effort compared to the baseline implementation of using `Option`.
I'm not a C programmer but I imagine you could make something like `std::optional` in C using structs and macros and whatnot.