vim-mergetool only supports scms that name their temp files BASE, REMOTE, LOCAL.

Svn names the files something like this:

file.cs.r404563
file.cs.r404217
file.cs
file.cs.mine

You should be able to start vim with something like this:

gvim -f -c "MergetoolStart" "%MERGED%" "%BASE%" "%LOCAL%" "%REMOTE%"

Requiring a specific order would make elements 0:3 of argv() those arguments in that order.

I think it's better to provide an scm-agnostic way to map the files, so maybe a variable g:mergetool_arg_order = 'mblr' that sets up those files could work.