War bonds are meant to be paid back, right?
They're simply issued IOUs with interest rates - debt securities.

The U.S. already issues many of those (and many end up in China).
So feel free to buy 'em.

The binding of income to purpose is an illusion anyway. Every additional $ bound purpose-bound income replaces a $ from the general budget. There's usually no increase in expenses by purpose-bound income, especially not in a situation as the AFG war as the source of income does not change the interests pursued with that war or its costs.