Am I correct in assuming that an obscure file format loader's c level source/abstraction that closely corresponds to a hex dump of the original file can also be used to make the said file format construction source code from scratch in what seems to be something like bootstrapping?
In general, no. There usually are auxiliary resources that do not need to be written out, but still have to be reconstructed by the loading function. It's hard to say anything more without knowing your specific situation.