if I can do commands like :

rcp file server://folder
rsh server command

then can I do :

tar options server://dev/rmt0 folder

That would beat having to create a filesystem to point to the drive of another server. The reason being having multiple backups per server with only one drive per server = one backup after the previous one and not all at the same time.

No, because of two reasons:

1. tar is not networking-enabled. It doesn't know anything about Sockets, IP, TCP/UDP/, ...
2. When operating on an hardware device (as opposed to a file) it interacts with the driver in order to get information about tape size, block size, ..., which isn't quite as simple when done over a network connection (which could drop packets or could fail)

Ok ..... how can it be done then ^

Option 1: Get a network-aware backup software. Those usually use agents to gather the files and send them to a central backup server, which then saves them to tape/disk/CD/DVD/...
Option 2: Temporarily export the root directory of the server you want to backup as read-only, mount it at the central server, run the backup, unmount, unexport