Big thanks to u/SixelAlexiS on Reddit for bringing this to my attention. Apparently (at least as far as we've both tested), this restriction seems to have been changed/fixed by Valve with the new Family Sharing update. And thus has made this petition obsolete. Glad to see this finally resolved. Especially for the people a lot more affected by this.

Their post detailing the discovery:

"About two weeks ago I had to add the Family Sharing on a new PC, and the only way to do that was to join the new system and so I did it.

We are back on today, where due to some recent news regarding The Wonderful 101 I've though "well, let's do another round at the game". I don't personally own it and I've played it back then via Family Sharing and I've always been sad about the fact that I couldn't play some big DLCs since they were free.

Well, once I've downloaded the game and started it, the free DLCs were there... all three of them.
I've checked the DLCs page where you manage them and they are there.
I've tested Resident Evil 7 as well, with the free DLC called "Not a Hero" and... well, it WORKS!
EDIT: I've tested Sonic Forces and free DLCs works as well. "

Thanks a ton to everyone who has either shared or commented on this issue, whether here, my Steam thread or Reddit post. I'm not sure if Valve or Gabe ever ended up noticing any of this discussion. But at least whether through our own efforts or just happenstance, this issue has seemingly been resolved finally. Can't believe this had been going for over 5 years... man time flies.

Regardless, Hooray! Huzzah! Banzai!

Thanks again everyone!