I think attaching a label, but worded slightly differently than your normal labels is a good idea. Something like "Pieced and Quilted or (made with love, etc etc) by (first name, possibly last initial) in support of (charity name) Include your city, state and the date (month and year of the auction would work if that info is known when making the label). I would attach it to the back in a way that the stitches could be clipped and label removed if desired by the person that purchases the quilt.
Labels do add value to quilts, so those purchasers that know this would leave the label on.