There are a couple more possible backdoors I can think of that might be able to install a "new EPG source" hack without opening the lid and without the need to enter anything complicated.
1) I seem to remember that some models of unhacked TiVo are supposed to be able to get a root shell on the serial port jack plug. Anyone know if this is possible on a UK S1, or does it need hacked rc scripts? If it can be done, then given a source of guide data, it might be possible to put together a little hardware device with a cheap microcontroller in it. Joe Public could plug it into the socket, and the device would issue commands to get a shell and from there reprogram the target phone number and any other PPP settings that might be needed. If it's possible it would be a zero-touch means of modifying units.
Of course, this method assumes there's a modem bank of some sort. Might just be possible using cheap/free SIP dial-in numbers (plus something like iaxmodem), if a TiVo modem speed of 9600 would work OK.
2) Some web sources say it's possible to enter a dial prefix that tells the TiVo to use an external modem connected to that serial port. Again, not something I have tried, and I have no idea if it's supported on a UK Series 1 unit. However you could envisage using this to give the TiVo a fake modem that's actually a small computer (e.g. a repurposed cheap router) that connects to a LAN on the other side to get the guide data. This would allow a network download of guide data without the need to install an internal network card.
Obviously this is all just speculation until we actually have a guide source. But from playing around with xmltv and Guide Tools over the past day or so, it does look like it ought to be feasible to put one together relatively quickly - so it's probably worth collecting ideas in the hope that we'll have alternative guide data before May/June.
As for database resets vs. ID synchronisation - if the price of continuing to use a TiVo is having to clear everything out, I'd have thought it would be one most people would be willing to put up with.