If you are using the 22 as a feature access code on the tie trunk between the two systems, why do you need to remove the digits? If you were able to get the Meridian to activate the message light on the Mitel sets, then you should be able to cancel the light the same way.
Is it the Mitel that is not working or is it the Meridian. If the process was working before, what changes were made after. Were those changes made to fix other problems to get over the software defeciencies in the Meridian? If so, then I would consider upgrading the Meridian. It is almost always best tech practice to keep your phone systems up to date for software. I know that Mitel software upgrades are free, you just pay for the labour of a tech to do it. Not sure about the software for the Meridian.