Well, I just spent a few seconds looking at it so I can't tell much, but so far, it seems that both use a very similar approach to store tags. Actually, Apple's tags appear to be OpenMeta reimplemented. Both use xattrs to store tags. If I am not mistaken, OpenMeta calls its attribute com.apple.metadata:kOMUserTags and Apple's tags are stored in com.apple.metadata:_kMDItemUserTags
So I guess if OpenMeta would use the same attribute name, the both system will be compatible automatically. AFAIK they already store the tags in the same format. One idea would be to write a script which runs through all the files and renames com.apple.metadata:kOMUserTags attributes to com.apple.metadata:_kMDItemUserTags