I suppose, legally speaking, only the commander can sign the tech log since he/she is certifying that any defects have been entered or "nil" as the case may be and that the times and details of the last sector are correct.
If the FO is dispatched to do any post flight paperwork and no engineers meet the a/c then there is a possibility that the Commander may omit to append the tech log and/or even turn the emergency exits lights off and/or the battery.
Normal practice is that the FO monitors the Captain. In the case where there are no engineers to meet the a/c then maybe it is prudent to retain the FO at the a/c to confirm/crosscheck that important post flight actions have been completed correctly. Otherwise it's all down to one guy and the human being who never makes a mistake or omission has yet to be invented.
However, I feel that from a practical point of view so long as the Commander can be contacted by phone to confirm the arrival details there is no reason why he shouldn't delegate the entry/signature to a responsible person.