Depends on how you define "callable". There are API end points to run KTRs and KJBs within the server. Same as the carte APIs. From an external web page, including the Ctools UI, you could submit and API call with parameters to execute a transform to insert data. GET and POST are supported, I would recommend using a POST. For a simple row insert, at the end of the KTR you could output the result as a JSON (or handcoded HTML response). If the KTR is more complex, you coudl trigger a status update and the original page looks/waits for that in the callback.
No examples on hand, but the concepts are well documented.