Emojis should be purely generic, and not represent an individual product or service.
No apple logo. No BMW logo. Generic.
Imagine if every brand had a fucking emoji, what a wasteland that would be. And then in years to come the unicode character space is littered with dead brands that have gone the way of the dinosaur but still living on in emoji form like digital trash, because emoji are permanent. There's no taking it out once it's made.
So I'd support an emoji to represent the notion of 'digital currency' but it should be generic, not Bitcoin.
There are some brand emojis in the Private Use Area, such as the Apple Logo and Twitter Logo, but they're not a part of the standard and thus aren't included in most fonts. Only Apple users can see this:
On Apple devices some installed fonts contain the Apple logo at this position. However, it is not recommended to use it in this way, since people with other operating systems will most probably not see this character.
The ConScript Unicode Registry suggests to use this codepoint for the Klingon Mummification Glyph. This encoding has been endorsed by the Klingon Language Institute.
It says enough about a person's worldview and education if they are serious about bitcoin being a viable currency. Specifically bitcoin. That was my point.
A lot of things exist in emoji because of early adopter advantage. A lot of the money stuff is either dollar or yen for this reason because they were first to the table. It's the same reason there are also so many other non-money emoji that represent quite uniquely Japanese cultural concepts in there.
There's a heck of a lot of world currency missing from emoji, but I would never suggest adding them all in. Everyone across the world is quite happy to use emoji with $ on as a synonym for 'money' regardless of what currency they actually use, so that already gets the job done as "generic money symbol"
It works because $ is so culturally ubiquitous.
When it comes to designing a new emoji that represents 'digital currency' we could take a similar approach as with fiat currency and just pick the most well known one (BTC) and roll with it.
But my preference would be to do otherwise, and design a non-specific emoji which represents the concept but without just being an existing logo.