Lord 255, you don't have enough pulchritude, or honor, nor do you have the eight set of Impetus Combs!
I am more of a Sleuth, though I am borderline Inspector!
I reject your systems and apply my own!
MAGUS TECH LEVEL 99 - GO!
*ahem* back on topic...
I still think addressing the stealth issues - or lack thereof - ought to come before devious new uses for infiltration.
While steadfast paranoia and constant placement of guard dogs are, and perhaps well should be, able to produce an ironclad defense against thieving kobolds and goblins, it seems like there ought to be tactics that WARRANT such paranoia. To elaborate on my previous mentions, if I may...
Rope Drops: Depending on how ropeladders are implemented next version, have kobolds (and other ambushers) employ them when possible; descending into chasms to infiltrate your fortress through a connected cave, for example, or use grappling hooks to scale an unprotected side of your tower megaconstruction.
Crate Disguise: Have kobolds sneak into caravans and/or find nearby crates, dump their contents, and crawl inside; when not observed, the crate would move slightly, and/or wait to be carried off as if it were the item in question.
Dirt Camo: Improved stealthing when in grass/on soil and not moving.
Immigrant Disguise/Shuttle: Would need descriptive paragraphs first to see if it's viable. If impersonation outright is impossible, then the kobolds could pay off an immigrant to ferry them in. This, however, goes into territory involving treachery, which is a huge beast of its own...
Lighting: Far future, but probably the easiest method.