Thank you for the alert on water buildup. I found this relevant article by noted frame builder Dave Moulton and this article by S and S Machine on the topic.

The Ibis tandem has one water bottle mount on the captain's down tubes, one on the captain's mid tube (for want of a better term), two on the stoker's mid tube, one on the stoker's seat tube, and one on the bottom tube. That's a lot of holes.

Given that adding S&S couplers is too expensive ($400 to $600 or more) to consider, what's the best way to inspect frame tubes for rust? Some recommend using a fiber optic borescope to inspect a frame. There are some flexible borescopes made with diameters as small as 0.6 mm that would fit through a water bottle mount hole, but they are quite expensive. I found an eBay listing for "Smallest Diameter Borescoe on ebay! 1mm x 6" + Light" but at a Buy It Now price of $1,275 it would be cheaper to get S&S couplers installed!

My guess is the best that one can do is to insert a paper clip through a water bottle hole, twist it around inside the frame tube, then tilt and shake the frame and see if any loose rust particles fall out. Any other ideas?

I have an old steel bike where the down tube, seat tube, and chain stays only vent into the bottom bracket. This is sealed pretty well from the outside, so I would think that since I don't see water buildup or rust in there then I wouldn't expect to see rust in my Ibis frame tubes. I still might think about adding rubber gaskets or Loctite to my water bottle mounting bolts to form a better seal.

Surly Bikes recommends applying JP Weigle Frame Saver, or motor fogging oil, or boiled linseed oil to the inside of the frame. I guess one could spray it through the water bottle mount holes if internal frame rust were a concern.