Daniel, I fear that ‘initial use’ is confused with ‘overall intent’. Yes, initial use was mapping a name to an address. I’d guess it is still by far the dominant use. But the statistics of use don’t determine the intent of the design. They merely make a case for basic efficacy.
Since its inception, the DNS has been the target of all sorts of additional uses. This gives credence to a view that the ‘intent’ was to permit other uses. (Not proof of it, but still…)
As for overloading by piggybacking, the description of the concept you provide is reasonable, but I’ll suggest it does not apply here. DNS was designed for a few different kinds of extensibility. It is not ‘overloading’ to use that. It’s just… extending.
An example of extending in a way that appears not to have been anticipated is, indeed, the underscore hack, which is a means of creating a clean mechanism for defining attributes to a name. Absent that, DNS node names have no type or other attribute information. While the underscore hack is ugly, it’s also effective. (cf, DKIM as an overlay for key distribution. Ugly but effective. And remarkably simple, compared with, say, DANE.)
“My post attempts to lay out the details of the piggybacking in this case, and argued as follows:”
You might want to review RFC 1034 carefully. I think it makes clear that the DNS was intended for a much broader range of uses than just name to address mapping.