The common and accepted practice among married Ashk'nazim, at least, is that they don a talis katan in the morning without saying a b'racha for the mitzva, and, later, when donning the talis gadol for morning prayers, they say a b'racha to cover both talisos.
But someone who borrows a talis gadol does not say a b'racha on it (Mishna B'rura 14:14, q.v.). So suppose someone arrives in the synagogue without having said a b'racha on his talis katan and realizes that he needs to borrow a talis gadol (where the loan is effected in such a manner as he would not normally be saying a b'racha on it). What should he do?
Should he, perhaps, say the b'racha on the talis gadol, having in mind also the talis katan (and touching it or moving its strings as in Shulchan Aruch 8:16, q.v.)?
This seems reasonable, as the talis katan 'deserves' a b'racha, but the current act of donning is of a talis gadol (which anyway, according to some opinions, requires a b'racha. I'm following the Mishna B'rura that in this hypothetical scenario one would not normally follow those opinions — but perhaps they should be allowed for).
Or should he say the standard b'racha for a talis-katan ("al mitzvas tzitzis") on the talis katan at that point?
This seems reasonable, as he never said it on donning the talis katan and the usual reason for skipping it — that he'll later say a b'racha on the talis gadol — turns out not to have applied.
Or should he say no b'racha?
This seems unreasonable to me, but I consider it somewhat likely because we're generally strict about saying unnecessary b'rachos and because I haven't found a source that says he should say a b'racha in this case (and the case doesn't seem like a very rare one to me).
 To clarify: Often, when someone 'borrows' a talis gadol, he actually gets it as a gift (to be returned), or is a part-owner, and may therefore say a b'racha. My question is assuming a situation in which the loan is a true loan, and asking whether the borrower's wearing a talis katan since the morning affects whether he says a b'racha.