See
Cross-platform feature support. According to this table "Dictionary lookup" is supported, but many indexing features and essentially all programming features are missing. There are often requests on the mobipocket forums for refunds because a dictionary did not work on a Java-based reader, but it isn't clear what percentage of dictionaries are effected and which (missing) features they are using. I used the iLiad as the Java-based reader example, but mobipocket always includes the Cybook as well. Bookeen seems to be using their own implementation, but I doubt it is more capable than other Java readers for dictionaries. So if a dictionary works on a Gen3 it is worth trying on a Kindle.
Note also that the table lists available capabilities which might not actually get implemented on a given device, the Kindle does not support multi-dictionary lookup for example (it can only have one "primary" dictionary at a time).
I also seem to remember a post about mobidedrm not working on a dictionary (but I can't find it). So the safest approach would be to find someone who is using the dictionary on iLiad or Cybook and confirm that they can remove the DRM. Then buy the dictionary and try it on the Kindle.