Since there doesn't seem to be an electronic solution to your conundrum I'll suggest a mechanical one...
Why not go to a two TM backup solution similar to joemike's, but with a small HD that you'd swap out for a new one each January 1st for the second backup (as opposed to his RAID array)?
You'll then have an ongoing backup and a yearly archive to protect against the truncation of the ongoing one.
Sounds like a reasonable solution. You could then put the small drive in a safe deposit box to provide a third level of security. Those
annual drives would be good for anywhere from three to five years before the magnetic fields would deteriorate to the point they become unreliable. (SSDs wouldn't last that long.) Which brings up an issue plaguing data storage technology, there is no reasonably priced media available for archival electronic data storage. The only viable solution at this point is to periodically
refresh the data by rewriting it on the media.
