The easiest way to achieve this is with an invisible and inactive button.
<axf:form-field field-type="button" field-name="tooltip1" field-description="Paragraph 1. This is the location for the tool tip text. Test." width="160mm" height="20mm"
Just omit the submit action and add the tool tip text to the field-description. You can then place this using an absolutely positioned block container with z-index set to -1 to make sure the graphic or text this corresponds to is drawn over the inactive button.
Download button-tooltip.fo and button-tooltip.pdf for a complete example.