Hello,
I proposed another solution that, I think is simpler. It does not the same thing, and it is based on your previous attempt: Dynamic Linking and Java - #3 by jevy.
Instead of packaging an (elf-)patched version of protoc-gen-grpc-java, I use the buildFHSEnv to emulate an FSH-compliant system.
The command described in the contributing guides are working.
I created a PR here.