I am aware of the HTML field, but Waindigo's Custom User Fields also adds a PHP callback for this section. I know you are not an addon developer so you may be the wrong person to ask for it. But, your skeletons have been super helpful to me so far, so I thought it would not be wrong to ask.
You have no idea how stupid I felt not thinking of that earlier. I was sure there was a reference somewhere I could get, and as much as I deal with outside PHP for Xen, this never occurred to me. So I have that working. Your ability to provide skeletons to some important things have alone helped...