BTC is a tool. Like atomic energy, it can be a source of clean sustainable energy used for peace, or be used for devastation. So no, BTC itself is neither legal nor illegal.

As far as your question goes.

Two things I need to know :

1. Whether the acquired citizenship is legal
(not the method) - would it withstand government / official checks?

2. What would be the cost of such a citizenship?
Considering the fact that you are categorizing sponsored citizenship as 'expensive' I am really curious to find out how cheap your way of doing things really is.

Of course, for the right group of people, this would be very beneficial. But since you're a tool (not an insult - refer to the atomic energy example). THIS can also be used for nefarious purposes. So I am not sure whether theymos or the moderators would allow you to offer such services based on ethical reasons as well as major political reasons.