Unfortunately, I cannot give you a specific literature citation for the load testers. The two magazines where I would have seen them are Radio Control Modeler (RCM), which may no longer be publishing, and Model Aviation, which is published by the Academy of Model Aeronautics (AMA) in the USA. You may have better luck finding an index for Model Aviation; although, neither magazine is very well indexed, and that statement is particularly true for advertisements. I have seen advertisements in Model Aviation in the past year for system monitors with real-time or stored data. Some of the motives behind monitoring are to detect control binding or excessive flight loads, which seem very similar to what you want to do. The ads would most likely have been in the section for new products.
I did find one construction article by George Steiner in RCM, November, 2001, pages 42-46. That project was a recorder for servo loads. At the time, his e-mail address was
gspprod@aol.com. My guess is that that address has changed, but George Steiner is (was?) a frequent contributor of electronic projects for model aircraft, so you may be able to find a more current address for him with Google. There may be other construction articles. I just don't have them indexed.
As far as an opinion, I think monitoring each servo would be preferred to monitoring all collectively, and then looking for the one drawing an excessive load. Assuming you have 12 servos, one over loaded servo, say with only three operating, may not add anything to the total current compared to having 8 servos operating normally. John