There are plenty of figures to use for Russian cavalry during the Russo-Japanese War, but not much for the Japanese. I have been given one suggestion, namely the Pacific War cavalryman with havelock. However I think the WWI Austrian hussar, cast with the pelisse removed, would make an excellent 1904 Japanese cavalryman.
Another point: I have just read that the Russian maxim machine guns in 1904-5 were mounted on light artillery style carriages with a shield, and not on the small WWI/II style carriage. Maybe the 1870 French Mitrailleuse? I've not seen the machine guns or pom-poms of the Boer War range, but would they be any good?
Alan/Shedman might be the one to answer that - his blog has a page on them. Two Marshalls, iirc!
There are undoubtedly better proxies for the Japanese cavalry than the Pacific War figures - I did look at the Austrian WW1 cavalry but thought that they required more preparation that the the ones I went for
And to be honest the Japanese cavalry rarely fought on horseback as they were no match for their Russian counterparts
Again for the early maxim you are right about the shields - again I bought a pack of the Mitrailleuse which would be ideal but as I already had enough WW1-style Russian mgs I didn't have the will to get them sorted
See (http://bergenschild.ru/Reconstruction/photosaloon/before1914/chronicle_tech/niva03_29_1905.JPG)
Votes added!