Alright, I played around with this for about an hour and decided that teleporting lights to a portal_detector IS possible, it is just extremely buggy and wouldn't work for a general case.
In order for it to work, though, you need to create an invisible model in front of each portal_detector. Using the portal model as a prop_dynamic instead of a prop_portal, you can accomplish this. Then you have to set up your portal_detectors to parent your light to this model, and then SetParentAttachment of the light to 'particles' which is an attach point for the portal model... also, the portal model has to be perpendicular to the wall...
Setting up two of these I managed to make a light move between them, but it does get slightly glitchy...
Example:
blue portal on detector_1
orange portal not on a detector:
Light shines through to the opposite wall of blue
Move blue portal to detector_2:
Light moves with the blue portal
Put orange portal on detector_1:
No light from either
Move orange portal back off detector:
Still no light
Re-fire blue back right where it was:
Light goes across from blue
As you can see.... this is not optimal, and still is not universal, as you would need to set up MANY portal detectors, and MANY portal-props to attach to, and even then... still doesn't really work if you put both portals on portal detectors (which would happen quite frequently).
All this said, I did only work on it for a short amount of time... there could be another way. Also, I've only been working in hammer for ~2 weeks, so what the hell do I know.
Best bet: set up spot lights in distinct locations and only have this effect work there by switching on/off the lights.