Clean it up really good and verify that it's where the leak is.
That "seal" is called a "liquid gasket". Basically, silicone. Kubota uses a lot of it as opposed to paper gaskets. You're right, it's not that easy to fix but definitely not impossible. You'll have to remove the ROPS, then the right side axle housing. The axle housing is bolted to the brake case. Then you'll have to remove the brake case, and re-seal it. The re-sealing process involves removing ALL (every trace) of the old sealant, from the channels and all. Then clean the surfaces with some sandpaper. Just scratch them up. This gets you down to bare metal. Clean the surfaces with brake cleaner, including ALL of the oil, and reassemble.
One tip....the bolts are different lengths. Make sure to put them back in the same place you take them out of. I haven't done a 2900 in a while but I do know that on the bigger standard L's (L4400 is what is fresh in my mind), if you get the bolts misplaced, and a long one goes in where a short one came out, when you tighten the bolt it can/will break through the back side of a blind hole. Same thing can happen if there's any oil or sealant in the hole. Another pointer, the brake plate is actuated by balls. The balls have a tendency to fall out when reinstalling them. This can be alleviated by using some heavy grease on the balls.
Sealant. I like 3M Ultra gray. It hardens up and it paintable as opposed to staying flexible and being nearly impossible to paint. I like to paint the joint after it's all dried up, as it makes for a more professional appearing repair, and if the owner (or myself on my own stuff) should decide to sell it in the future, it will look factory and is one less question for a potential buyer.