Thanks again to all for the advance info. "The rest of the story" is that we did head down to Big Bend NP last weekend. With a cold front pushing down from the snowstorm our home state of Colorado was experiencing, the weather down there was mercifully cool and nicer than we were expecting. The campground was pretty full, and the site we got was only available for two nights, not the three we planned on, so we had to try for Boot Springs the first morning after we got there.
Up at 5, hiking before six (in the pitch dark with headlamps), we started at a good pace on the Laguna Meadows trail. Being old and decrapitated, our pace slowed along the switchbacks. We thought we heard at least one Colima along the trail when we first gained the plateau at the meadows. In retrospect, this was correct. More time there might've saved us a couple of miles.
We did finally get a nemesis bird up there, at the top of the third set of switchbacks, that had been tormenting us with constant announcements of its presence, and finally showed itself for a few minutes-- the Black-chinned Sparrow. A thrill to get that monkey off our back, and add it to the Varied Bunting as the second lifer of this trip-- our fifth to Texas to bird.
Having gained the Colima campsites and started to descend, we saw a mixed flock of warblers, and heard another Colima on the way down to the Boot Springs/Pinnacle trail junction. A couple on their way up told us they'd just watched a "pair" (2 males, actually) of Colimas by the old barn, just South of that intersection.
So we hoofed it over there and found the birds! We watched them for 10 or 15 minutes until a guided group of 8 or 10 people showed up, spread out, unpacked lunches, and started chatting, immediately driving the birds back into the brush. They did eventually become active again, affording us more great views of this hard-earned lifer.
As bernerjc mentioned above, the Pinnacles trail down was a total knee-burner. In restrospect, I wish we'd spent the extra easy mile back uphll on the Colima trail to take the Laguna Meadows trail back down. Would've been much easier on the joints. But getting the bird definitely took some of the sting out of that!
We also saw the Black Hawk on its nest at Rio Grande Village, the White-collared Seedeater in Laredo, and the Bronzed Cowbird and Groove-billed Ani in San Benito. Missed out (again) on the Aplomado Falcon. Good trip in all, though, and 7 new lifers!