Sorry, I didn't know that --disable-splash no longer works, and have currently no way to check if that's indeed the case or not.
As has been mentioned, at least on Windows there is the very real chance that external DLLs are being "pulled in", which is something that is commonly checked using a tool like "Dependency Walker", which brings up a list of DLLs that are utilized by an executable.
That being said, I do believe that --disable-splash should continue to work as is, if it doesn't that would qualify as a bug - it was originally added for a reason, and I see no reason to remove that option. So if you are sure that the option no longer works, I would file a bug report or raise this on the devel list. But again, don't file a bug report unless other people hav e independently confirmed that the option no longer works.
Technically, it's not really obvious to me why this part of FlightGear has been revamped at all - it could have been trivially replaced using a single Nasal/Canvas wrapper that shows arbitrary raster/vector images and osgText using modern OSG 3.0 code instead of yet another bunch of C++ code added to the initialization sequence. As a matter of fact, the canvas-team clone provided the exact functionality 5 years ago, at the mere cost of initializing the Nasal scripting interpreter earlier, so that the basic Nasal/Canvas building blocks would be made available while the rest of the system is still booting, which even meant that the Canvas-based "Aircraft center" could be executed during the initialization sequence:
http://wiki.flightgear.org/Initializing_Nasal_early