You can only have real form buttons. Either by specifying the tile, or by giving a deform.Button object.
What I need sometimes is to have some arbitrary html in the form where it has the buttons (for design reasons mostly).
Right now my workaround is to move that html next to the buttons with some jquery after the page has loaded - but what I really want is to have the controll to just render arbitrary html as one of the buttons.
You can make a new novalue field lied to an htmlwidget which renders what you defined, either a plain string or maybe even better the rendering of a chameleontemplate.
If you make a proper pullrequest, i would be happy to make it go into deform itself
FWIW, the deform master is growing features that should allow you to generate forms "from scratch" (aka it is growing a "retail" API), so it's likely that we'll ask people who need highly customized form output to use that rather than building more knobs into deform itself.
This feature is now documented (minimally and poorly) in the "retail.rst" document in the "docs" directory of the repository (and will be up on readthedocs soon).
Here's the full link to that documentation: https://deform.readthedocs.org/en/latest/retail.html