Mid-span ethernet monitor

I need a device that can sit midspan (at a "splice point") and monitor
an ethernet connection for integrity.  At the very least, that continuity
exists to both ends of the span.
While these are PoE/PoE+ drops, I don't want to place any significant
burden on the PSEs; an external power supply is therefor likely.
I can tolerate brief interruptions in traffic (e.g., during "testing")
and even continuity (less desirable but tolerable as the testing is
only sporadic)
Active, in-line solutions are complicated by power delivery over the pairs.
So, I'm wondering if I could passively monitor signals coming in/out and
communicate that status through a different channel?
Cost isn't too critical.  Though physical size might be.  (the wet-dream ideal
would be something that could fit in a 8P8C coupler)