Casascius coins are likely considered a collectable and while IRS regs and the tax code theoretically allow collectables in an IRA, the practical matter of actually doing so is much more difficult.

For an IRA to be considered real and recognized by the IRS it must be confirmed to be compliant.   Major custodians like Fidelity, UBS etc. have no problem at all getting the plans considered to be complaint....but there are almost no major custodians who can take non standard assets like this.

So....you'd be looking at either a high end very specialized solution done through a law firm or family office....usually something focused on investors with over $50 mm......or you'd be looking at getting your own custom IRA to be considered compliant.   

Both are hard paths.

Ideal solution for Bitcoin is something which is on an exchange and requires no special approval....one on the exchange ANY IRA which has access to stocks could buy it very easily.