is it possible to set a variable (like a version) in the main BUILD file or pants.toml and use that for other BUILD files in my monorepo? I want to have a single place to define the version and I thought that might be a good place.

See https://www.pantsbuild.org/docs/reference-global#build_file_prelude_globs or, if the shared value will be used for a target field value see https://www.pantsbuild.org/docs/targets#field-default-values

Thanks!