One solution consists of using memset to initialize the structure to zero, and using memcpy to copy it instead of structure assignment. (Problem: pass-by-value in function calls won't use memcpy; abstractly, it uses member-for-member assignment which is not required to copy padding. A function that wants to calculate the correct has has to prepare a blank object with memset, then individually assign the fields into it from the incoming object.)

Another solution, more along the lines of what I was thinking, is simply to associate the hash table with a hashing function which processes the type as a structure, hashing the members individually rather than as a pad of memory.

C++ templates refine this by adding the ability to deduce the hashing function statically, and possibly inline it, which we could do with some preprocessing in C, along the lines of how those TAILQ macros from BSD work for linked lists.