this with the Humax 5400 is very interesting. I own one myself and may do the conversion some time in the future.
But, what would be more interesting for me is a conversion for the old Hauppauge DVB-S cards (Technotrend, Siemens, ...). I run a big video server/recorder with 4 of those cards under Linux and Linux-VDR http://www.cadsoft.de/~kls/vdr
and it would be great to get SDI output for this. Onboard a AV7111 is used which pipes the data to a SAA7146A. It should be possible to collect the BT.656 data at the SAA7146A because there is no direct access to the pins at the AV7111 because of packaging.
At the same time I also think about the addition of a homebrewed SDI input to a Lumagen Vision with SIL504 chip. May be one of the interfaces used for Vigatec can be missused for this. With an additional multiplexer it should be possible to integrate this. Silicon Image provides schematics for its starter board which exactly does this.
But this projects have to wait till winter time. ;) Currently we have here more than 35 degree C just like in Spain and it is hard to do productive work. :D