I do agree with your statements on Mistborn only being Invested when actively burning (as in, other then their innate Investiture, they can only hold kinetic Investiture), but their spiritweb still contains the connections to access the Investiture, and therefore it is somehow as if they were Invested. They act like they are Invested because of these connections, I mean.
Here's a longer WOB on that (I split it into the two spoilers below so I could comment)
So here he says that yes, they are not specifically Invested when they are not burning the metal. They are Invested when they are burning it. So far so good. Here's the rest (SH spoilers):
So it seems to be from this that Invested isn't the right word, but because of their Spiritual connections, it is still more difficult to, say, Push/Pull on them.
So to answer the original question, I think that the answer is well yes, but actually no mainly because "Invested" is the wrong term, but a being with those Allomancy sDNA connections... the universe is going to treat them as if they were partly Invested.
Now the question becomes, when do you gain those sDNA connections? Because I realize if they gain those connections upon snapping, then they're screwed. But if it's part of your sDNA from birth, then yes.