Why don't you want their information to sync to SFDC right away? It sounds like that's exactly what you're planning on doing manually anyway?
You could create new Marketo fields for Name, Email, etc that you put on the form. This way, you don't have to worry about the sync because the data will live in a separate field. Then, whenever you're ready, you can export the data from the new fields and put into the standard ones.
We don't want the information pushed it right away in case the data is sloppy. For example, we wouldn't want their Company Name to update if there's a reason we have them in the system a specific way. Our customers, for many reasons, may be referred to by multiple corporate names. Your solution may work - I'll test it out!