This issue is already implemented. As the comment from dboreham suggests, PRI ALT has been done with the "simple approach" — it's parsed, has a Priority flag in the AST (ast/ast.go:392), and generates the same select as regular ALT (since Go has no native priority select). There are both parser tests (parser/parser_test.go:384) and e2e tests (codegen/e2e_concurrency_test.go:357).
The TODO.md already marks it as Implemented. Issue #84 tracks the potential future "faithful" implementation with nested selects.
This issue can be closed. Want me to close it with a comment?