I would definitely consider integrating directly with Marketo instead of with SFDC, unless you absolutely need the data in SFDC. I've worked with several clients who've created their own "free trial" signup form, and then immediately pass the information into Marketo. Marketo then sends a thank-you email to the registrant (which is immediate, while if you go via SFDC, you have have at least 5-10 minute delay because of the sync).
The easiest integration method is a server-side form post. This means that your server-side application (the app that handles the free trial) will re-post the data into Marketo. Any web developer should be able to set this up in an hour or so. They post to the same URL that is used in a native Marketo form (usually https://app-x.marketo.com/index.php/leadCapture/save, replace x with your pod), then specify the field names using Marketo's API name (use "Export field names" in Admin > field management). Also include the fields 'formid' and 'munchkinId'. I can provide more details on this if needed.
So in this approach you continue to use the forms that you are using today, we just send the information into Marketo using a scripting language (PHP, ASP, Java, etc.).
Hope this helps,
Jep