Well at least one obstacle has been removed - the TiVo Service Number that is used to get your data slices in each daily call. Unless it was a "02300060..." (ie a Thompson Scenium TiVo) model the servers could just refuse to issue guide data even if you'd done all the work of hacking into your later series unit, making all the PAL mods, etc. You'd lose RGB scart and widescreen switching of course.
However there would be more obstacles to take its place - for a start the guide data for later software versions than our 2.5.5 would have to be slightly differently formatted (just going by all the code branches in TiVoweb alone), doubling or tripling the workload of providing data. It's not impossible - probably yet another item on the "some time in the future maybe" list
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links to my TiVo logos, TivoWebPlus 2.1.b3 UK-20120818, Tracker v3.3.3 & v3.3.4 (17th Jun 2013), GDchecker v1.06b, Digiguide checker v0.4.3-rc5 and Autospace v1.65 can all be found in
this post.