TP.org accepts -pre1 etc. and -rc1 etc. versions. I see several of them in the package list of the translation project. On http://translationproject.org/html/maintainers.html in using the TP is more info. In the past Ben had some objections against using the -pre1 etc. postfixes. I am not sure anymore why and don't know if the same reason still exists.
I think we did do this onece before, but as I recall, it did not bring in a lot
of translations.
At least the -pre is used a long time ago. It was last used for version 0.7.4.
I believe several of the other translators read this list, maybe they can jump in and give their view. If almost nobody would use it, it is almost useless.