I found the problem and solution so I'm posting it as a reply to my own post in case the original post turns up on a search.
I owe a big apology to Struts, the problem had nothing to do with Struts. As far as I can tell Struts 1.0 has no problems with Japanese properties.
The problem turned out to be the properties file itself. The properties file was created using MSWord and the encoding that resulted was not the same as used by the rest of the application. When a tag asked for a message using a particular key, that key matched in all properties files but the Japanese file. I'm guessing that the bit
pattern used to hash the messages was different for the Japanese file so nothing in the Japanese file ever matched the keys in the application.
The solution was to recreate the properties file using the NativeToAscii tool. I would think any re-encoding would have solved the problem.