For those interested, an update:
-First, thanks to all who replied, great ideas. I'm impressed with the skill of this community, and the overall tone of the site. Fwiw, I come from a technical climbing background, some of those sites can be downright snarky.
-I went ahead and bought Leafield's air gauge. A bit pricey at $30, but I suppose I have an inner geek streak, I wanted measurable data (not to mention I wanted to know exactly how tight 2.5-3lbs psi was).
-After practicing a few wet exits with the new boat at a local reservoir, I deflated it and took it home. Reinflated it in the basement to fully dry it out. This time when inflating the problematic left tube, I thought (?) I heard a barely perceptible click in the valve when inflating. Thinking something might have been lodged in the valve, I did a few more fast/hard strokes with the K Pump, hoping to fully dislodge anything in the valve.
-I then fully inflated each chamber and carefully measured it with the gauge: 2.5 psi for the floor (the pressure relief valve kicks in above 2.5), then 3 in each of the side tubes. I then let the boat sit for almost a week.
-I went down this morning and remeasured each chamber. The floor was exactly 2.5. The "good" left tube was down to about 2-2+, but the "bad" right tube was just above 1 psi, even though it felt pretty darn taut to me. Way, way better than the original problem, which had a few visible wavy wrinkles in the outer fabric.
Bottom line: Fluid testing in the valve didn't show anything, I think the leak is just too damn slow. I'll go ahead and use it this summer, it works just fine (going out to do the Moab Daily on Friday, fwiw), and I can top it off easily for multi-day trips, I carry a pump anyway. But I'll contact Aire once I shelve the boat in the late Fall and see what they say. Thanks all for your replies,