I've made an extension for Internet Explorer 8, FormToAccelerator which turns HTML forms on a web page into either an accelerator or a search provider. In the design of the accelerators format we intentionally had HTML forms in mind so that it would be easy to create accelerators for existing web services. Consequently, creating an accelerator from an HTML form is a natural concept and an extension I've been meaning to finish for many months now.
This is similar in concept to the Opera feature that lets you add a form as a search provider. The user experience is very rough and requires some knowledge of accelerator variables. If I can come up with a better interaction model I may update this in the future, but at the moment all the designs I can come up with require way too much effort. Install IE8 RC1 and then try out FormToAccelerator.