I looked the code for FV50 and in subscreen 1010 there is a standard check for mandatory fields (including 'ACGL_HEAD-BKTXT'): include MF05AFMENJ, form manadatory_fields. It is customised in T003 table for all document types. In case some document type has t003-xmtxt flag set, then it'll be required that the user fills document header text.
BUT: I am not specialised in FI so it is necessary to find out what other consequences such customisation may have. For example, as I just found, one of the places using this check requires filling document header text in MIRO (Logistics). It's better to get a consult from FI and, maybe, LO specialists.
That's what I can suggest from standard.