I am not sure if this will work but you may try using extra hidden fields in the task form as well:
<input type="hidden" id="REQUEST_MY_FIELD" value="REQUEST:EXTFLDV_500221">
<input type="hidden" id="REQUEST_MY_FIELD_HTML" value="REQUEST:EXTFLDV_500221_HTML">
Then it could be possible to pass these values to your query (not sure about the query for inserting task history record though):
:lHISTORY_DESC[REQUEST_MY_FIELD + :lREQUEST_MY_FIELD],
:lHISTORY_DESC_HTML[REQUEST_MY_FIELD_HTML + :lREQUEST_MY_FIELD_HTML],