I'm jumping on a old thread here, but my perception is:
You can think of the key as either B minor or D major (or switching between the two). Most of the negative lyrics are in the verse (Bm home) or land on minor chords, whereas the chorus is based around the relative major (D as home). The chord preceding the chorus is A (the dominant of D) which strengthens the feeling of D now being Chord I in the chorus. This A chord is also the major chord under the word "light" - which like the perfect cadence into D, also leads us from the negative (minor) section of lyrics (and harmony) into the hopeful lyrics (and harmony) of the chorus.
My analysis here: https://docs.google.com/presentation/d/1Y9dIocRBrvM-LXgfQZf41-5Mm6lVoLiV0CInbTrRQdc/edit?usp=sharing
So, if we analyse the chords in the key of D major (which this section is based around), then when you get to the bit (I think) you are referring to you have the chords:
D, Bm, G, D.
I, vi, IV, I.
When you get to G (IV) you have a plagal cadence from IV to I (G to D). When sitting on chord IV (G), which I think is where this C creeps in, it sounds "better" because C is the subdominant of G. So you kind of have a secondary subdominant implied there: C->G->D, coming round the circle of 5ths in fourths. Plagal->Plagal.
So, in summary I perceive the C to be borrowed from the subdominant of G, which is the subdominant of D (the key, or at least home of the chorus): VII->IV->I. Like how in lots of music you often see the extended cadence II->V->I rather than ii->V->I.
The VII->IV->I extended plagal cadence is common in blues / rock and pop etc. which is where I believe this sneaky little "C" comes from.
This is a good explanation of the Beatles doing this explicitly with the chord progression itself, whereas in Could You Be Loved the "C" is just borrowed, or the sub-subdominant is implied: