On many 2000-2005 Bonneville the center reflective lens separates at the top and allows water to fill and stay in the center lens. So much so that when full the trunk opens only halfway, or may not even open. The proper repair is to buy a new center lens- which can be very pricey. A hasty fix is to simply drill two small holes at the bottom of the lens (as seen in the pic). Takes less than one minute and it works just fine. If you take a close look at the pic you can see the final drops of water leaving the lens- when holes were fist drilled was a very steady stream.

Is this happening in such a way that removing the assembly, separating it, and then resealing it is not feasible?

I don't know. I'm not familiar with the 2000+ body parts.

Drilling holes in lens bottom corners is a great idea, reminds me to do extra holes in the bottom of my doors before fall too.
Drilling the hole relieves the water, but may also allow the lenses to fog up inside come winter, if the seals too are not addressed.
I had to remove all my rear lights, & then seal all my Pk Ave'* housings as they were all fogging up bad in cold temps.

I resealed mine by running a bead of silicone along the top edge of the lens.

I pulled the GXP taillights out (sold them on Ebay) and put in a old set of SSEi tail lights. The center tail light section I installed was taking water for so time, the opening across the top was wide and long- I suspect must fixes would of failed after so time. Since the center tail light was already in below average condition, the one minute solution of drilling the holes seem to match my goals of not having more damage and not spending a lot of time.

BTW- the Bonneville GXP tail-light frenzy seems to have died. Took me a while to sell the tail lights (last one sold two days ago) and at less than half the price they were selling for a few years ago. I would even speculate a SE/SLE/SSEI tail light is more sellable today than a GXP tail light.