You should be able to use the Humax as a set-top box feeding in to the TiVo providing you can find the IR codes for it. As a test, try changing the IR setup on your TiVo and see if there's a code for Humax - you want code 20073 (medium speed).
viewtopic.php?f=8&t=1904&p=19792&hilit=humax#p19792