Secure webform inside non-secure page
You shouldn't really try. If form is served via HTTP, attacker already stolen your user's data using MiM attack to inject his own JS code into your site, so sending it via HTTPS is pointless at this moment.

If you need reliable security, always serve form using encryption. If you don't, don't waste resources on unneeded SSL processing.

See Transport Layer Protection Cheat Sheet for more details, as Clive suggested.