Making modding more legit is quite a large topic. As your all aware and garry has said it himself, it would basically fall upon the devs to implement all the functionality we are expecting.
Only way I can really see modding getting intergrated without increasing the workload for devs that much, is maybe pick a bunch of community members to sign a NDA or two and get them to work on the API voluntary. If this means they get dummy files that simply consist of all possible function calls, structures for information they need to try to create a API then it can only be a good thing.
Granted id like to admit any chance for supporting clientside mods id be highly interested in, as at present I've reframed from doing any as VAC will get upset. be nice to make fully custom UIs etc..
I too believe a well written API could be exactly what the modding community needs, a structured environment to go wild in. But as you mentioned, increasing the workload for devs is the most undesirable approach especially during such a crucial phase as alpha.