That sounds like a bit of challenge. For the record, there are now over 50 published papers showing facial grimacing in animals caused by pain, and grimace scales have now been developed for rat, cow, horse, cat, sheep, lamb, rabbit, pig, monkey, and ferret. In fact, what may possibly be wrong about the original paper is the idea that grimacing only occurs for 24-48 hours. A number of groups have now shown facial grimacing in a chronic pain context, and we ourselves are starting to see that with longer habituation, mice with SNI do indeed grimace.
This being said, I also would be happy to hear about people’s experiences with the MGS or RGS. Software to automate the laborious process of turning video into still photographs is available for free, and as stated above, machine learning algorithms for automated scoring have been developed by multiple groups.