added a comment - - edited
Implementations of com.sun.mirror interfaces is clearly impractical, and anyway unnecessary for a processor written for 269 compliance; only exotic processors cast to these interfaces, such as Lombok (which is unnecessary for Groovy anyway). Dealing with Groovy language constructs which cannot be clearly mapped to javax.model also seems out of scope, except possibly reporting such cases as warnings. Extensions to Filer seems unnecessary since any processor using such a feature would already be Groovy-specific.
My request would just be that if you take a Java class which is successfully handled by a 269-compliant annotation processor, including (in general) generation of Java sources which then trigger an additional compilation round; rename it to use the *.groovy extension (assuming it is using no Java language features not supported by Groovy), as in the sample project I initially attached; and compile using default options; then the same annotation processor is run as before, with the same results.