Silly me... of course the field names has to be in the brackets:
<input type="hidden" id="REQUEST_MY_FIELD" value="[REQUEST:EXTFLDV_500221]">
<input type="hidden" id="REQUEST_MY_FIELD_HTML" value="[REQUEST:EXTFLDV_500221_HTML]">
Still you have to update these fields dynamically with javascript before submitting the form (I see you have managed it already).