I, too, think that Elizabeth would not have been able to issue a false confession. Yes, she lied to save her husband's life and reputation (or so she thought), but throughout the play, Elizabeth demonstrates that she is extremely concerned about her relationship with God. She tells Rev. Hale that she is a "covenanted" woman, meaning essentially that she has lived a life that is holy and that she expects to go to Heaven because of her "holy agreement" with God. Lying to spare her life would not fit into Elizabeth's diligent pursuit of "holiness."
Similarly, by the play's end, Elizabeth--like her husband--has grown. She has come to see her faults in the way she treated her husband and to recognize the poor opinion she truly had of herself. With this self knowledge and renewed love for her husband, Elizabeth would be strong enough to resist trying to spare her life; for at the play's end, she is strong enough to resist trying to convince John to sign the confession (even though she surely wanted to spare her husband).
I would take a different view, in that I do not think Elizabeth Proctor would confess. We have no indication throughout Arthur Miller's play that Elizabeth is anything other than a good, pious, Christian Puritan woman. Just as Rebecca Nurse cannot bring herself to lie in order to save her life, I don't believe that Elizabeth would have been able to either. She would lie to save others, as she did in court when she lied about why she fired Abigail, but not to save her own life, in my opinion.
John would surely have tried to talk her out of it, and would have gone to the court and admitted his adultery just as he did, surely done anything to get his wife out of the hangman's noose. But in the end I think she would have wanted to be an example to her boys of how to live a Christian life, even if it cost her her own.
In my opinion, Elizabeth is not near as strong of a character as John Proctor is. She would likely confess so she could live but still remain in jail. Who knows, she could be counting on the fact that with boys running around the magistrates might eventually send her home to raise them, not to mention to go have the baby she is pregnant with right now.
I also think John would convince her to confess with the reasoning that if she just remained alive, they would be able to figure everything else out later. John has worked so hard to prove himself, I think he would continue that quest just to help her know he could and would take care of her.
Elizabeth's weakness as a character is demonstrated thus far throughout the piece as she has let John do what he could for her. She also lied for John once, I think she would do it again to save her own skin.