Not sure if this is of use to anyone however I thought I'd share it as it seems to be working for us so far.
Basically, I've managed to get a Z800 HP machine to read and write to tape using Acronis software (£80) and a small, single drive Quantum LT04 Half Height tape drive (you need an SAS board in your PC).
P2 Files are logged on P2 Viewer (£0!), with 'proxy' virtual P2 cards being created, unplayable but they do have a thumbnail image derived from the Icon folder, adding the Archive tape number to the information so in the future you know which Archive tape the shot is sitting on.
Before anyone says - 'why not just use external drives', a box of 5 LT04 tapes is £100 and will store 4TB of data, with each tape having a shelf life of 30 years.