If I understand you correctly, you would argue that "right and left"
refers to places of honor in Jesus' answer because he did not have
the authority to grant it. However, Jesus goes on to say that they
"belong[ed] to those for whom they have been prepared by the Father."
I would say that Matthew's (and Mark's) audience was surely familiar
with the Is53:12 prophetic proof-text ("and he was numbered with the
transgressors") as some MSS of Mark attest at 15:27. Thus, Jesus'
comments can be understood to mean that those places at his "right and
left" had been prepared at least as far back as Isaiah the prophet,
if not earlier.
-- Stephen C. Carlson : Poetry speaks of aspirations, email@example.com : and songs chant the words. http://www.mindspring.com/~scarlson/ : -- Shujing 2.35