I was on the phone with my dealer when I read your post and took the opertunity to ask him. He told me that he sold the same model as your with tooth bars on them all the time. Two last week for a father and son and he said he had one in their shop right now that was getting a new tooth bar installed.
My dealer said that as far as he knew there was nothing that would void your warrenty but if you wanted more info to call Kubota and get a answer direct from them. If you were closer to me he said to bring it to him and he would put the tooth bar on and still have the warranty to. He said that he had sold 10 or 12 BX25 with tooth bars installed and had never heard of a warranty issue.
My personal thought on the matter is that if it goes against the warranty why do they offer a tooth bar for that model tractor. The one that my guy sells he said just fits to the bucket and has 2 bolts that hold it on and you are ready to go. It's hard to know which way to go with two dealers telling you completely different stories so maybe Service Dept Vic or some of OTT's other tech's will jump in and help out.