For a euro brace you are probably going to have to drain the tank. You may not need the silicone to cure water tight but I'd at least want it dry to the touch if it were me. I have never done this.
The alternative fix for a running reef would be to replace the black plastic center brace with a clear acrylic strip. You can do that with the tank running, but again I'd drain it as much as you possible can first. Then cut the brace and bolt it in with nylon screws. I did this in 2007 or 2008 and my tank is still holding water fine but I was scared. The clear acrylic still casts a bit of a shadow but nothing compared to the black plastic.
This is not my picture, one I found on a google search but same idea :