Pete,
I don't know if this will help, but when I go through material I pick up in collections or at auctions, most of the cancels that are difficult to find in Schimmer's book are not missing, they are just not listed for the issue. I rarely run into items that are shown not to be found. With that said, this may be the reason that these Not Yet Found cancels are still either rarely encountered by most of the collectors or have even been identified as NYF items from most of the few people that actually watch this web site.
Your best bet is to collaborate with Steve and search though Mexicana for added notes on found cancels. I would bet you would come up with the vast majority of any cancels that can be found. Now the tough part is being able to disregard a lot of the listed cancels that can not be found that have been erroneously listed.
If indeed anyone does do a remake of Schimmer's book I would suggest that they consider a more user friendly set up. Since cancels listed by states rather than districts seems to be so counter active for the collector use to Schatzkes set up with cancels listed under district rather than state it makes it much harder to find what they want. This is especially true for those stamps that we have with district names or consignment numbers which is of course the Banknote issues and the Foreign Mail issues. Later issues have cancels that are normally found by the names on the cancel, thus needed an index to find them anyway. Yes, I know that the district names were not anything one would consider for the later issues, It is either divide off the earlier district stamps and place them back into an expanded Schatzkes and make a new book covering the later issues by state, or use the districts as a starting point to find the cancels.
Anyway, good luck and I hope you or Steve don't give up on this project. It is very much needed.