I think U can set the storage units groups,

put 2 drives only, and use it for all policies.

For example:

here we have 6 drives, 2 of them R reserved for Archieve/logs 

The problem is not with tape drives - how will you manage and separate media?
You can separate the drives by adding only two tape devices in NetBackup.
You can have different pools in NetBackup and add O/S media to pool DONOTUSE.
O/S backup will not be able to send mount request to tape library.

I can just imagine if someone accidentally mounts a NetBackup tape in an O/S drive to be overwritten by tar....

tar can be used as any userid, but needs read access to all files/folders that need to be backed up. Write access is needed for restores.

if you set your storage unit to only use 2 drives, then NetBackup will only ever use those 2 drives, leaving free the other 2 to use for your OS directed tar backups.

As for your user question, I'm not sure if tar and all of the associated commands to write to tape from the OS are open to all users, but you could certainly set up a sudoers file to control access to those commands for different users

This is based on what I know of MY LIBRARY.

I create a virtual library called library_a I only put 2 tape drives in it.
this library_a is added to netbackup
First only add 2 tape drives to netbackup.  When adding tape drives to NB you tell it what library drive number it is.  So drive 1 and drive 2 get added to netbackup.
So now NB only knows of 2 drives.

When I put tapes in the MAP (media access port) I tell it the tape belong to library_a.
So when I do an inventory from NB it sees the tapes in the MAP because they are assigning to the NB library and they get moved and the NB database is updated with the location.

Now my library has web access.

I add tapes to the MAP and I say they belong to the system - library (MEANING I HAVE NO WAY TO TRACK THEIR USAGE)
Now I use the web interface to move a tape from storage slot 112 to drive 3.
I do my tar stuff from my os.
when done I eject the tape from the drive and move it to the MAP and I take it out of the library and some way tag it so I know what it is)

This is just a theory - not tested - just what come to my mind when I read what you wanted.
Maybe someone else can improve on the thought.

i am aware that tape library drives can be zoned at the drive level then you can then zone one of them to either NBU or OS.

This section is from NBU admin guide page number - 231, hope this helps you to use only 2 drives out of 4.