You can simply download it. (rotmg site/AssembleeGameClient[version].swf) current version is 1344965814. It will join only rotmg testing. ( silly protection, that can be patched with one byte)
Hah it's seems to be easier than i thought. You can make marinepowers work with testserver in very simple way. All you need is to edit dnsmapping.cfg and add correct server adress. You can get server adress from [rotmgsite]/char/list just look for <server> tag.