JMO, you're referring to the second inversion G-Bb-Db-F (on third half note)? Isn't it really a 4/2 dominant in disguise resolving to I6, which seems to be your assertion?
Is this a big deal?

I definitely hear it as Charles described.
Schenkerian analysis or not, it's obvious as a V-I gesture, just inverted.

Big deal, anyway... WWMD? (what would Messiaen do?)

Well, some people are silly!

It seems impossible, in my experience, to use a root-position dom7 effectively in chant accompaniment. Inversions are rare too, but in the right context (like above), voices that are moving fluidly might land on a tritone. It's what comes before and after that seems to make the difference between it sounding like modal accompaniment or a harmonization of a hymn tune.

In this example, it looks like the author was driving at a descending bass line, and the smoothest voice-leading for the other parts created this half-dim/dom7 harmony by coincidence - but the "problem" was beautifully framed by what is before and after, and we really don't hear it as a jarring harmony since the voices all move so fluidly.

Something's causing my laptop to double post. Must be because this thread is for theory geeks. Ya can't Telemann how to Handel his Basso Continuo, so I'm going Bach to bed.

I hear it as one big unfolding of Ab. The triad in the opening of this example is unfolded via V (the Eb triad), whose bass passes through Db into the first inversion Ab triad; the "Ab-ness" of it never really goes away. In my opinion, the whole passage should be understood horizontally, not vertically.

The "interesting" note here is the F neighbor, which is an augmentation of the top line's neighbor motion that occurs *three times* in the passage. Very clever. The composer could have simply held the Eb across, but as Ryan noted above, the F seems to mitigate the "dominant-ness" of the passage. And you wouldn't get the neat motivic relationship.