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Instead of being cryptic, I'll just straight-out ask for helpsies.

I wired up an ethernet loop-back onto the end of my flash-drive. I want the lanyard portion to be a Cat5 keystone jack.

But, I also want to jam an LED and a 3V battery in there (I'll work out placement later), so that when the loop-back is separated from the keystone, the led lights, but when the circuit is closed with the loop-back, the led extinguishes.

Is that even remotely possible?

Sounds like something a PNP transistor could be used for? Using a transistor as a switch, an NPN blocks current from the collector to the emitter pin (off/closed) and switches "on" (allows current to flow from the collector to the emitter) when there's a voltage on the base pin. A PNP works like a switch but the other way around (current flows until there's a voltage on the base pin). Maybe you could use this to allow current to flow through your LED when the physical connection is not present?