I'm not aware of any HD OTA converter box that can be controlled via the serial port of the TiVo. The HD OTA converter boxes are all controlled via IR, at least the ones from the converter box coupon program. The TiVo knows how to control those boxes.
You can get IR blasters from several sources. I know the ones from Xantech work, or you can get one directly from TiVo.
Hope that helps,
1+ Pioneer DVR-810H-S
1+ Pioneer DVR-57H
1+ Toshiba RS-TX20
1+ Humax DRT400
1 Humax DRT800
Most with Lifetime and upgraded drives
All Series 2s with Linksys USB200M