I have sent CD booklets before to Sarah and she returned them, signed, quickly so I decided to send a piano to her to sign and sure enough....she returned it signed also.
Sarah is real good for TTM items. Unless she is out on tour, she has returned things back to me, signed, very quickly.
Carole King, from what I see on this link, still signs thru the mail and seems to be happy to. I found she always personalizes but to me, it doesn't matter for somebody like her.
I got my Sarah McLachlan autographs several years ago and from what I see on this site, she seems to be signing a standard 5x7 of herself now more often than not when you request something from her. It seems that years back, when I sent her things, she was signing whatever you sent her. It might be than now that the cat is out of the bag she tends to not to anymore. I'd say, if you do send her something don't send her something that would upset you if it didn't come back but got something of her choosing instead.