In Minecraft Vanilla SMP, having access to the server, can I somehow retrieve the position of an offline player?
Yes. Use NBTedit:
Editor by copyboy from minecraftforum.net:
Download it, and run, click File -> Open then go to your Minecraft server directory and find players directory and select the name of player you want, for example Player.dat
You should see something like this:
You need to go to "Pos", and there you have it, coords of player you wanted.
Just double click any to edit it :D