This is highly doubtful as SAI will not be selling directly to the public, but through a dealer network. So you won't be able to communicate your special order to SAI, who will be very busy pumping.out 200 rifles per.month anyways. I highly doubt that the factory would entertain special orders such as yours in the first place. The R18 Mk3 is not a small-batch gun made to order for each customer like the Raven.
Your best be will be.to sell the TriggerTech that comes with the R18 and replace it with a cheap milspec trigger mechanism after the fact.