Why not buy the bonds direct? I qoute from treasury direct:

"You may already participate in a 401(k) or other similar thrift plan. It's wise to make use of these plans, designed primarily for retirement. These plans, however, usually have some heavy-duty restrictions or penalties if you pull your money out early. I Bonds can be used before retirement, if necessary, helping to protect your retirement investments. I Bonds can also supplement your other retirement savings.

There are a couple of ways you can use I Bonds to save for your retirement. One way is to start saving regularly today and defer paying taxes on the interest that your bonds earn. After retirement, you can cash bonds to supplement your retirement income and report the tax-deferred interest as income at that time (when you might be in a lower income tax bracket)."

No point in having them in your IRA.
David