Yes, you could write a property source that would SSH into the server and get the max_connections from the config file and set it as a property on the device. Then you'd need to slightly modify the mysql global stats datasource so that it used that property in a complex datapoint that uses a formula like this:
if(un(##max_connections##), threads_connected, threads_connected / ##max_connections##)
I haven't tested this, but this is the gist of how it would work.
There's an example Expect script here: https://github.com/sweenig/monitoring-recipes/blob/master/DataSources/Groovy/Expect/Groovy_Expect_Example.groovy
