I have not, no. However, the Saphir products seem to be reliable. Otherwise, I have only used turpentine to strip the finish on my CXL boots. It works, but you have to really work to rebuild the finish. My guess is that the Renomat is a bit milder. Otherwise, I am not aware of any other products for stripping wax or polish.
I actually have dipped into shell. I picked up a pair of black shell cap toes from Alden NY. But I returned them for a different size which is on order. I concur with Uncle Mac. Try the damp cloth and vigorous brushing. If that doesn't work, then exploe other options.
Phipsi, have you ever used that stuff? I wonder if it's safe for cordovan? I know you have yet to delve in to the realm of shell. You recommended it too me over the summer as something that could help take the meltonian shoe polish off my Dad's cordovan loafers. He covers up the natural beauty of the shell with the stuff, and part of me wants to help him out, but the other part tells me to just let the man do as he pleases.
Uncle Mac, do you have any guidance on the matter?
Edit: Looks like Hanger Project says its safe for cordovan, but than warns to test it on a small hidden portion of the shoe, as it can be too harsh for some leathers.