Hi Chris,
The general answer to "Can I cross-post fields from a non-Marketo form to Marketo as well as to the form's original endpoint, to create/update leads in both places?" is an emphatic "Yes."
And if you're in control of both forms, it's extremely easy to hook a visible form together with a hidden Marketo form, just a few lines of JS.
But it would be reckless to give a thumbs-up to a specific third-party forms library (i.e. an embed which your company does not control) without analyzing it first, because it is possible for that "Yes" to change to "With difficulty" to even "No" depending on how their form logic is implemented.
For example, some forms libraries are very robust in terms of internal features, but they don't distinguish the two events Attempted to validate and submit form and Successfully submitted form (i.e. they know the difference internally, but don't have a robust external event model). As a result, it's impossible, or significantly complex, to figure out when you may cross-post the data to another system (be it Marketo or any other) because the initial form is not conclusively telling you, "OK, I just put/am about to put my data on the wire, now you can mirror the same data if you want." It's only telling you "Someone clicked a submit button"* and it's possible that the internal form validator will reject the form post, but too late, you've already mirrored it.
In sum, I'd say it's most likely going to be possible to do what you describe, but someone will have to look at how their forms fire events.
*This area is, by the way, why a completely code-free integration between Google Analytics and Marketo is impossible, since you need to make sure the GA hit only fires in the Marketo onSuccess, not on-basic-HTML-form-submit.
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