Thanks for the reply. At least I know I hadn’t missed something basic.
I can live with basic radio options for field B, and I suppose I can figure out for that myself the html to be sent from ajax. One issue arises from that though. On initial page load, there would be hundreds of field B options, which would badly break the page layout. Any thoughts on how I can restrict the vertical height of the radio options list? Is there a way to wrap the field in an iframe, for example?