Hey Sam.
I've worked with WORM tape a bit. WORM disk (like SnapLock) is easy enough to integrate - I'm working on one of those systems right now. I don't think TSM supports encrypting disks on its own though. I haven't gone over all the documentation for TSM 7.1.x. (Which is now called "Spectrum Archive" -- I suppose it was overdue for rebranding...)
If TSM doesn't support encryption natively in the latest versions, IBM bought a company with a product called 'Vormetric' that did filesystem-level encryption, although I don't know what they call the product today... I'm not sure if that plays well with SnapLock though.
Long story short, it's going to be an interesting challenge. If you can find NAS hardware that supports BOTH encryption and WORM locking of data, then you're set... Otherwise, it's going to be a bit of a mess.
Please keep us posted!
-JD.