Historically after Bernie left band, pretty much all of the country/bluegrass parts (e.g. banjo on Midnight Flyer, pedal steel on The Best Of My Love) were played by Don F in live performances. Joe's hands being too big would partially explain this in case of instruments like the mandolin, although I find it hard to imagine him playing the main country parts of those songs in any case. Would this also explain why Don F played the steel guitar on But, Seriously Folks...?
(As an aside, one thing I've noticed is that in the 1974 Don Kirshner's Rock Concert version of Midnight Flyer Bernie is of course on banjo, with Glenn on slide guitar and Don F playing acoustic, but in the 1976 Houston performance it was all change, with Don F taking over the banjo, Joe playing the slide and Glenn on acoustic!)