The way I was pursuing (but got distracted) is to change the /forms/standard.adp for radio. I added this check at the button to insert the textfield. This assumes the widget is called "radio_other". The widget itself would just call "input radio ....".
<if @elements.widget@ in other>
<input type=text name=\mailto:"@elements.id@.other\">
</if>
Sadly, I did not get the chance to test this thoroughly (previously I was using a hack, where I hardcoded the "other" element as the last one of the options and did a check whether the label was named "other").
If you continue down that road, great, I'll be back on track on monday.
