According to this the tern post diameter is 33.9.
http://www.ternbicycles.com/us/forum/seat-post-size
Here is a Woodman topper designed for integrated seat masts that's available in 33.9
http://www.woodmancomponents.com/Web...%88%2005:46:21
So, provided you are already not at the post's maximum extension and have a few cm to play with, hack off the end of the stock post and install the topper and saddle.
If you don't want to alter the stock post you could always buy another in the appropriate diameter and do the same. Here's a 33.9 on ebay for $25 + ship.
http://www.ebay.com/itm/New-TOTTA-SP...-/150604266924
A longer railed saddle like a Fizik Arione or Tundra might also work to get you the setback you need. Here's a side view of a Tundra. Long rails are long.
