The donation of miles is dicey from a tax-deduction perspective. You'd effectively be trying to make the argument that you bought the miles as part of product bundles with airline tickets or other products. Not sure I'd want to wade into that who mental/legal exercise with the IRS. For all I know, somebody's already done it and lost.

However, it would seem to me that if you donate an entire airline trip directly to a charity, you could more easily document the fair market value of the trip. Still a gray area...I haven't done it myself...but at least the documentation and explanation side of things is more straightforward.

I see people donating hotel rooms to local silent auctions all the time: from the fine print on the items, I can usually tell what the underlying certificate is. A Marriott Megabonus certificate, a credit card annual free night award, etc. Usually there's a FMV listed on the certificate (sometimes realistic, sometimes inflated), so the donor is likely writing that off on his/her taxes.

(IANAL. Or an Accountant...)