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.
Annual swaps are certainly feasible, but if one drive dies in its year, your other one may not be too recent. It is possible to swap drives on a daily, weekly or monthly basis. The interval on which you do it would be the most time your data are out of date. This does not require a RAID configuration; just tell TM that each drive is a backup and it does the rest automatically.
