AFAIK, the Veritas SBS serial will only be accepted if the media server is on an MS SBS machine
Since you have a Veritas SBS serial and a Server serial, yes, you could backup the SBS box to Disk located on the XP box, and then backup the BKF files to tape with the standard license.
If you ever want to restore, though, you would need to resore the BKF files, then inventory the B2D folder, then restore to the original location
If you have the Disk and Tape backups run on the same box, you can DUPLICATE the B2D to the tape device and then be able to restore directly from the tape.
You could also purchase and install MSSQL and Exchange Serials on the XP box with the BE Standard serial EBay usually has some one and two generation old serials listed.