At the risk of causing some confusion I would like to add that I did get them to show up (all four versions from bunyap's pack), but only when I selected the A-4 Skyhawk. EDIT: Also with the stock A-6A and A-7A/B, but not with the TMF A-7E (2nd EDIT: that's because of the A-7E's ARM stations max weight limit)
Okay, I think I have it now:
The reason why the AGM-78A-C don't show up for the F-105, no matter whether the attachment types match or not is because of the "Exported=FALSE" line....since in-game USAF and USN are treated as different nations. The F-105 is regarded as an USAF bird and can therefore get only access to those USN weapons that are listed as Exported=TRUE.