Is anything like this available (or planned)?
No there is none per se.
But you could use your own form development system and use Marketo server side API to pass the lead info to Marketo. See Marketo REST APIs: Lead, List, Activity, Campaign Objects » Marketo Developers
What exactly are they trying to achieve?
You just post forms directly to http://<your-marketo-domain>/index.php/leadCapture/save2
Just ensure you have all of the necessary hidden fields:
Along with any other fields you want and it'll just work.
Unless you actually have a large, successful campaign, in which case it'll fail because of rate limiting.
This is why client-side forms are what everybody should be aiming for. Server-side form posts are now deprecated in favor of background client posts using the Forms 2.0 API.
Both the Forms 2.0 and Landing pages built within Marketo POST their form data to /index.php/leadCapture/save2
Yes, but if you are talking about server-side form posts they are rate-limited (in fact, client-side form posts have the exact same limitation, as it is per-IP, but needless to say it's less likely that users sharing the same NAT IP will encounter the limit than that all your leads together will fail).
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