It would be nice to have an educated, well thought out, and reasonable estimate of the amount of time it would take for a large fraction of users to have the BTC symbol work (and for someone to fill in all the assumptions with reasonable guesses)...
There's no way of avoiding years of time and effort. It's either a system related issue of failing to implement existing UTF-8 to get the double symbol to work or needs a single symbol (with both strokes) added to Unicode. Either way we'll be waiting years before 80-90% have moved on to systems which will support it.
The alternative is to use ฿, Ƀ, ¤ or some other symbol until a consistently displaying independent symbol is available to everyone without relying on the web or customisations. As much as Luke-Jr rails against it, this is exactly what will happen.