Sure, a webhook can be used to report form data anywhere. (HTTP POST w/standard form encoding is one of the supported formats.)
You should be aware that if Eloqua has a limit on form posts from a single IP, you are potentially creating a DoS vulnerability by sending all your posts from your Marketo pod (this is the mirror image of the problem when you use the Marketo forms endpoint from an external server, in that all the requests come from the same IP or small set of IPs).
A webhook posting to an Eloqua form would be the easiest. There are lots of companies doing server side form posts to Eloqua in large volumes and I would not worry about exposing a DoS vulnerability - doing it via any integration other than posting from the client side, you will be in the same boat.
Other approaches using APIs (and code running outside either system) include:
1. sending to Eloqua through its API
2. setting up a cloud decision step in Eloqua to query the lead in Marketo through the Marketo API
When I talk about DoS, it's not about volume in general, it's point-in-time rate, and it's about maliicous + legitimate use.
If Eloqua has no limits, that's great. Marketo doesn't. 43,200 form posts per day sounds like a lot until you have irregular legitimate distribution (which everyone does) let alone a concerted hacker (which everyone will).