Hello,
Right now there is no such plugin, and I don’t even have two days experience with gradle (or groovy for that matter), so I thought I’d share my work.
I have added automatic META-INF/services/* file generation in my build script. If you want to have a look, it is here:
https://github.com/fge/msg-simple/blob/master/common.gradle#L67
Note that in build.gradle, the processResources task depends on this one.
Have fun!