Have you got a really good friend you can trust with anything?
If so, write the letter and address it to the OW. Post it to your friend with details of how to contact the OW. Tell friend to tell you she HAS forwarded on your words to the OW and never speak of it again.
Schrodinger's OW. You neither know, or don't know if the OW has really read your words. If this helps, then you know for sure that the motivation for your contacting OW is to "get things off your chest" and was motivated by a desire for closure. If it doesn't help, then you know this was never your motive and in fact, what you really want is to gloat that you got the "prize" and she didn't.
An alternative way to do this is to write the letter as a MN post and post it. For all you know, OW is a MN'er and will read it, but you don't know that for certain. You'll probably never find out if OW has read your post or not, again, Schrodinger's OW and the effect is the same, you'll at least discover what your true motivation for wanting to contact her is.
I don't think this is your problem though, I personally think your anger is misdirected. You've decided to work on your relationship and stay together, but to do that properly you have to forgive your husband fully, not just re-direct your anger to the OW instead because it's easier than facing the fact your husband cheated on you. Until you can forgive him, you'll always want to direct your anger at OW and if you feel like that will never go away, you probably can't forgive him.
Good luck OP 