I'm creating a solution to deploy a list definition and to go along
with it, I need to add an extra button to the EditFormToolbar (and
the same one to the NewFormToolbar).
I've created a CustomAction feature which places the button
correctly, but the UrlAction does not seem to be working correctly
(or is different than I would be familiar with).
All examples I could find of this type of CustomAction use the
EditFormToolbar button to open a new page/application (aspx). Instead
I need to execute some JavaScript.

When I put JavaScript into the UrlAction tag of the element, the page
renders the link with something like href="javascript:__doPostBack()"
which reloads the page, though it does add my JavaScript to the
onClick method of the link. I've tried adding "return false;" to my
JavaScript to cancel the PostBack without luck.

Anyone have suggestions for going about solving this issue?

As a side issue, I'm deploying a JavaScript file which this button is
going to call a function in, but can't find a way to include it in
with the NewForm/EditForm without actually changing the form.aspx - I
was hoping to use some trick JavaScript to load it in with the
UrlAction, but clearly am having issues with any JavaScript in the
UrlAction.