Object defaults are for the properties of newly placed objects, yeah.
At best, it would be Document Options, or Layer Stack Manager (you can change layer thicknesses and counts, however!), but libraries just don't have that many options. You might think they adopt the design rules of the project they're linked from (if included in that way), but that doesn't quite work, because design rules are specifically within the PCB. (They could've done it that way (a really long time ago, like PCAD days probably, or earlier even?), that is make design rules a project scoped setting; that would've made supporting multiple PCBs [within a project] harder, but I mean it was a long time before they had that as a feature too. Well, turns out they put design rules in the PCB, so that's that.)
There probably is a hidden set of rules or object defaults somewhere, but embedded in the program, not normally accessible to the user. It's probably not in a file, I would guess, but if you want to flip through zillions of files, who knows. (Offhand, it's not in the ADVPCB.DFT, which only stores objects listed in Tools/Preferences. Bonus points if you're able to create whole new object defaults from scratch though, lol.) I wouldn't know where to begin with that, but possibly some extents can be probed with scripting, without resorting to attaching a debugger or whatever.
Would be nice, but yeh. My annoyance is, I prefer to set non-plated holes/slots as zero dimension copper, which makes them ~unselectable in PcbLib view. (I set a NOT HoleIsPlated rule, soldermask expansion, from hole, +0.08mm or so, to solve this once it's on the PCB.)
Tim