Patching iMessage on legacy devices is a quick and simple process, which can take 10-15 minutes. I have done this myself on an iPhone 4S running iOS 6.1.3.
This guice does not require any jailbreak, just patching certificates online.
Patching process
If you haven't yet, patch your certificates. If you have already done so, skip to step 5.
If you're on iOS 5.0 and up, click "Signed iOS Bundle (iOS 5+)". Otherwise, press "Unsigned iOS Bundle". This is for patching Apple ID logins and HTTP/HTTPS issues.
Press "Install" all the times iOS tells you to.
Go to the Settings app, then click on iMessage. (NOTE: This guide has been confirmed working without a SIM card. All you need is Wi-Fi.)
It should ask for your Apple ID + password again. If it says incorrect, it still should work, but I recommend double-checking your password first so you don't have to restart the last 2 steps. This time, you MUST input your password with the 6 digit code sent to your other Apple Devices/phone number AFTER your password. (bitetheapple4036 to bitetheapple4036XXXXXX)
Wait for the login to finish, and the iMessage switch should be automatically on. Under it it will say "Waiting for activation...". iMessage has already been activated, and this line can be ignored. That line will be there FOREVER, no matter what you try.
iMessage should now be patched and working! Enjoy!
Apple JB