And another thing
Just a reminder.
SMS is not a guaranteed delivery mechanism, just because person A sends person B a message does not mean they will receive it. Not only that it is not instant, near instant if you are online, but if you are offline you receive it when your operator feels like it. iMessage does at least update itself shortly after you go on-line again.
I'm surprised you can't use iMessage through an iCloud app, though, which would be visible from Windows, or indeed any other browser.