CLI/API for exporting custom data groups

When creating a module/extension with a custom-data group, one needs to produce a file (e.g. an XML file) with a description of various CustomGroup, CustomField, OptionGroup, and OptionValue records. One can currently export all custom-data groups with bin/migrate/export.php, but the resulting XML has way too many records.

Two possible formulations:

1: drush cvapi --ij CustomGroup.export id='["IN",[1,4,7]]' return=xml > my-data.xml
2: civix generate:customxml 1,4,7

#1 requires adding more features to the drush interface – e.g. support for inlined-json arguments (--ij) and support for somehow output "print $result[xml]" instead of "print_r($result)". This seems like the more flexible option, but I'm not clear on how to make the output work.

#2 seems easier and is now the leading candidate unless someone has a clever idea for how to handle the output.