Thanks to Adam Schiff and Ngoc-My Guidarelli for their helpful contributions. It turns out that the relationship of the Modern Language Association of America to the American Council on the Teaching of Foreign Languages was as sponsor. I didn't know that a sponsorship relationship would be expressed in MARC using a subfield b as is done with hierarchical relationships. Perhaps this was a pre-AACR2 practice. So this has been a learning experience.
I adjusted the NAR to change
4102 Modern Language Association of America. ǂb American Council on the Teaching of Foreign Languages
5102 ǂi Sponsoring corporate body: ǂa Modern Language Association of America ǂw r
and added a 670 to document the source and the history.
To my knowledge, there's no way that Gary Strawn's Authorities Toolkit would be able to distinguish between a hierarchical relationship and a sponsoring one. So it's up to the cataloger to check. (Gary advises to check in all cases of toolkit use.)
Catalogers using the toolkit also need to remember, when adjusting an existing NAR invoked by clicking on a controlled field in Connexion, to add a 670 field for the source bib record that prompts the edit. This is easily done by opening the NAR in Connexion, then immediately suspending it (Alt-S), followed by closing the NAR and re-invoking the toolkit. You can then do "Edit--Create 670" (Alt-E 0 [zero]) and the toolkit generates a 670 based on your source bib record, which you can edit further. If I'm missing an easier way to do this, I'll be grateful to know about it.
Sincerely - Ian
Cataloging and Metadata Services Librarian
George Mason University