I have the same interest (it seems to be a common question in the forum) and I posted a similar customisation question here ..
http://orbeon-forms-ops-users.24843.n4.nabble.com/extending-formbuilder-tutorials-td4655153.htmlAlso in Form Builder Tool Box you can go to Advanced > Edit Source Code and add custom code there and Apply. Does this meet your requirement?
http://wiki.orbeon.com/forms/doc/developer-guide/xbl-components-guideI've found DOM Inspector and Web Developer and Firebug Add-Ons in Firefox to be useful in inspecting the layout of xforms controls. After installing go to Firefox > Tools > Web Developer > Inspect and then hover over the controls in browser view
Another approach is to intercept HTML code post Publication stage (after HTML is generated).
I'm experimenting with adding jQuery controls.