What are the legal restrictions on a Credit Union buying and holding a digital asset such as BTC or BTSX?
How much would it cost to buy a Credit Union that is already up and running? Examples of recently sold Credit Unions or ones on the market today?
Is it simply a matter of becoming the largest depositor in a particular Credit Union as they are in theory owned by their members?
1) for legal restrictions, see NCUA Rules and Regulations Part 721- Incidental Powers, for additional services with respect to pre-approved activities permitted to carry out a credit union’s business purpose. It's quite restrictive, but maybe can work.
2) You can start your own. http://www.ncua.gov/Resources/CUs/Dev/Pages/Start.aspx
3) I don't think you can "buy" a credit union, since the formation and entity is governed by ncua, which in turn requires credit unions to remain whole entities. The board exists to serve the members (depositors). Each member gets one vote, regardless of deposits - so you can't just "buy" it out.