not withing 24 hours officially. However if you go to the station from where you want to board, the station staff might be able to help, no guarantees though.
I offer this suggestion because of a recent experience at ALD. Some passengers from south India who were on a pilgrimage tour had return reservation tickets from Mughalsarai to Bangalore but then got delayed in ALD. So they talked to some railway staff at ALD and they were told that a message has been communicated back to Mughalsarai and they would be able to board from ALD and their seats will not be allotted to anybody else because they didnt board at Mughalsarai.