You can capture IR codes for a new set top box using the instructions here:
IRSliceCreator
You need to get your hands dirty using telnet but it is possible.
I have created codes for a modern NeoStar Freeview Box ( A mini scart Freeview PVR for £30)
that seems to work well. Get one Here
The receiver has a 5 volt supply so it may well be possible to power it directly from Tivo.
The attached TCL file contains the IR codes for this receiver CableBoxCodeNum = 29992
This file contains the IR Codes for The Curtis DVBTUK-T3 Freeview box available from Tesco for £20.
CableBoxCodeNum = 29992