No, I dont think there are any recognised UML only certifications.
What IBM are certifying is that you are an OOAD developer -- that is more than just the 486 exam.
They feel that you must also be a
java programmer (155) and have
IDE skills (282) -- in fact they also offer options, and you can decide to do Oracle exams instead if you dont like IBM (see jCert).
Your question is like asking if you can get a 1 year degree -- the value of the certification is made up, in part by the dificulty in obtaining it.
You should also ask yourself why you want the certificate -- if you want a piece of paper saying you know UML then sit the exam, and get a score report.
If you want to be a better developer, study for the certification -- you will surely improve.
If you want a valued qualification, then follow the jCert path.
James