For everyone interested, I made an experimental tree-sitter parser for OpenACC directives to provide a minimal (directive) syntax highlighting for Neovim: GitHub - loicreynier/tree-sitter-openacc: OpenACC grammar for tree-sitter · GitHub
I chose to create an external parser rather than contribute to tree-sitter-fortran as the syntax highlighting could be used with parsers for different languages via injections.
This idea was briefly discussed in an issue on the latter’s repository: Highlight openmp statements · Issue #164 · stadelmanma/tree-sitter-fortran · GitHub