Force the owner and group for the contents of a tar file?

Linux doesn't use internally owners and groups names but numbers - UIDs and GIDs. Users and groups names are mapped from contents of /etc/passwd and /etc/group files for convenience of user. Since you don't have 'otherowner' entry in any of those files, Linux doesn't actually know which UID and GID should be assigned to a file. Let's try to pass a number instead:

$ tar cf archive.tar test.c --owner=0 --group=0
$ tar -tvf archive.tar 
-rw-rw-r-- root/root        45 2013-01-10 15:06 test.c
$ tar cf archive.tar test.c --owner=543543 --group=543543
$ tar -tvf archive.tar 
-rw-rw-r-- 543543/543543    45 2013-01-10 15:06 test.c

It seems to work.