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will give you a better idea to as exactly how small that portion of the mullion is:
Having said that you could certainly add to the mullions the Trine 4800 series electric strike as seen here:
assuming of course you have not already installed the exit devices and can account for the marginal adjustment of the backset requirement. We do keep that strike on hand and can ship it out quickly. Please advise the voltage and type of current along with finish.
Alternatively I would look for a different device that can provide you with electric latch retraction as I would like to avoid having to run low voltage down through a million that is removable. I think long-term having to disconnect power to remove the mullion would that be an ideal solution.