It's a brilliant idea, and I hope it gets somewhere, but the issue then becomes where do you draw the line?
In this instance, they've given 10 weeks+ of notice. That's more than enough for practically every human endeavour, and given that you can only buy advance (cheap) train tickets only 12 weeks in advance, and the tickets aren't on sale for the match yet, hardly seems unfair when viewed objectively. Obviously some people - like the fans from Berlin - will be significantly inconvenienced, but to an extent that's luck of the draw. They could have delayed flights, or cancelled tickets, or there could be flooding, whatever - all things that could mean the match was put back with no-one to "blame".
Bury made it through to the 4th Round last night - for example - and as a result our away game with them at Gigg Lane has been "re-arranged" (read: cancelled until they know how far Bury are going in the Cup). That's with far far far less notice. Alright - Bury from Sheffield isn't that far in the grand scheme, but imagine if Southend had made it through and we were due to play them in a week or so on a Saturday? Isn't that just the same problem, albeit not caused by Sky? As YES Blade and Darren point out - "All fixtures are subject to change".
As I say, don't get me wrong, hope against hope it works and some charity gets a decent donation, especially as I've no love lost for Sky at all. But I think you'll probably struggle.