I've come across only one source where the issue of stability on the leg is addressed, an essay by Graham Wade titled "Some Thoughts on Posture and Holding the Guitar" (EGTA Guitar Journal no. 1, 1990), but it deals only with the left leg (which is not a problem for me). For what it's worth, here's what he has to say about it:
"Carlevaro suggests the use of a cushion on the left leg to prevent sliding, the cushion being made from sponge or slip-free material such as latex. The problem of the guitar’s ability to slide freely is a crucial matter. Many players become accustomed to the guitar’s inherent mobility and actually move the instrument slightly during recitals and practice sessions. Others, particularly those who employ Carlevaro’s concept of a sponge cushion, or Dumond’s foam rubber friction holding the instrument, become used to a more or less fixed guitar which cannot shift and around which the player’s hands move rather like a pianist’s hands over the fixed plane of the keyboard.
In such a situation the positioning of the right arm becomes less significant as a means to the guitar’s fixture during playing, the hold being taken by the legs, not the traditional points of support. Similarly the left hand is then not tempted to hold the guitar in place in any way. Some players, sometimes without realising its full extent, do use the left hand as a way of stabilising the guitar and inhibit its manipulative functions by so doing."
I've experimented briefly with a sponge on my right leg, but found it to be insufficiently effective, as it tended to move. As for other slip-free materials, I'm concerned about their potential to damage the shellac finish; if anyone has used a non-slip material successfully with a French-polished guitar, I'd like to hear about it. My preference, though, would be to find a solution that involves only posture modification.
Any advice would be appreciated.