I have spent time over several days unsuccessfully trying to locate my copy of "Eye of the Needle" (which incidentally I see is recommended in the link you provided) but in the index there is a copy of the Jewish Observer article which you and your source have mentioned. My recollection of the article is that it didn't merely mention this idea in his name but rather the whole article originated with him. The origin of the article is explained here "Rav Moshe Feinstein zt”l, in a call to action that was directed to yeshiva students and published in the Jewish Observer in June, 1973". Using one's time to lehrn Torah with the less capable is also also mentioned in the Igros Moshe:
אך הוריתי שכל ת"ח אף שצריך לעצמו והוא גדול מאד מחוייב ללמד מקצת זמן גם עם אחרים אף שמתבטל מתורת עצמו...ומסתבר לי שהוא ג"כ שיעור מעשר עשירית הזמן (אגרות משה אה"ע ד:כו(ד)) ץ
On Shabbos I noticed that this idea is mentioned in the Stone Chumash:
R' Moshe Feinstein stated that a Jew should tithe not only his possessions, but also his time, by contributing time to the service of worthy causes. (comment on Genesis 28:22)
It is not clear that their source is independent of those mentioned above.