Been there done that.
A M16 bolt can accommodate a 11mm hole (with a M12 bolt-stub plug), which fits a microSD card inside. (Actually, the microSD just does not fit, which can be alleviated either by enlarging the hole by a fraction of a millimeter, sanding down the card's edge a barely perceptible bit, or milling a slot to the side of the hole.)
The plug on the end of the bolt is barely visible, and for longer-term storage it can be obscured by putting the bolt in service and painting it over.
Older test with smaller bolts here. The removable-head one is less practical than the kind with plug at the end.
I should've submitted it! Too bad my machining skills trump my public relations/self-promo ones by a wide margin!
(bangs head against the nearest wall)
edit: Just realized the smaller one with removable head could be improved by a small hole drilled to the side with added lock-pin. Then the bolt could be screwed into some existing construction and unscrewed later (as the head would not unscrew until the lock pin is removed).
Another thought: a microSD card hidden in the HEAD of a bolt. The M16 bolts are too big for most indoor home uses. A bolt with hollow head could be made.
I also think the main-article bolt is from the same vendor that sells hollow coins, many of which can accommodate the microSD card too.
Edit: Here's the guy. Bolts at the end of the page.