Product Variations are also cms.sku objects, just with a "SKUParentSKUID" not as null (if it's a number, then that ID is the SKUID of it's parent)
So just look for /rest/com.sku/all and try to find the variant sku information in there, you can also access that object's SKUAvailableItems to get the inventory.
Likewise, if this gets a bit difficult, you can always use a Custom Query Repeater to query the SKU with some URL Parameters, Transforming it into a JSON object, and then use the Webpart to API converter to convert it to a JSON feed. That's how i do my APIs and i never have to touch code.
http://devnet.kentico.com/marketplace/web-parts/webpart-to-api-converter