Obviously, we can use an API to send data to Marketo...
Bad idea anyway...
We were hoping to do pre-filled and progressive forms, hence the reason we prefer to do embedded. It also gives us the flexibility of changing the form without going back to our developers every time. We also do not want to send duplciate leads into Marketo.
This is very, very simple if you have a competent developer. Simply use the Forms (JS) API -- that's the client-side API and must not be confused with SOAP or REST APIs -- to relay the same fields to your external service in an onSuccess listener.
It's the same concept as sending, say, a hit to Google Analytics in the onSuccess, you just happen to be posting all the form data to another service.