The way to make the user IRs work is not obvious, so some people may have tried and failed due to that.
First, you have to put the replacement file within the folder named REmatrix, not REmatrix Player.
Second, within the folder REmatrix, Overloud has placed a file named userIRS.rir, which leads one to believe that it is the file you should replace with the file of the same name from the REmatrix Solo folder, but it is not. This file is in the wrong place to have any effect. (It also has no IRs inside of it.) The replacement userIRS.rir file must go inside the IRLibrairies folder, alongside the file REmatrix Solo.rir.
I have also noticed a couple other differences between REmatrix Player and REmatrix Solo. The equivalents to the Time and Stereo knobs in Solo are disabled in Player. That's somewhat of a deal-breaker for me.
I still think it was great that Overloud did as much as they did for Sonar users.