Select Related Entries from a specific category in CP
There is no native functionality for this at the current time.

You could write a plugin to extend the Products field-type (which itself extends the base entries field-type) - to provide this filter functionality.  That is you'd add a categories chooser to the field type and then filter the available products using that category in your plugin.

https://pluginfactory.io would get you started on the plugin scaffolding, and I'd look at /craft/plugins/commerce/fieldtypes/Commerce_ProductsFieldType.php and the native Craft categories field type to get started.