It is not exactly forbidden, but there are problems that can't be solved by the casual user.
1) The RTP elements can only be used with an official RM engine, if your game uses another engine you'll need complete custom content.
2) currently the RM engine works only for Windows, and the EULA forbits reverse-engineering for other systems.
Some game developer solved this by creating a windows-emulating wrapper around the unchanged engine to make it run on other systems - but there are cases where the RTP is used with a different engine on other systems, and that is usually illegal (to my knowledge there has been only one case ever of a licence given to use the RTP without the engine, and due to rumored problems with that deal Enterbrain doesn't give that option at the moment.
However, an official converter is currently in Alpha-stage and has been tested on one single game released a few months ago. The developer whose game was used for that alphatest reported that the converter still has problems and is not simple to use - the entire project had to be configured differently from the PC version.
I don't know how long it will take to make that converter usable, but I don't expect it to be fast. Perhaps we might see a public beta by the end of this year (yes, I know it's January only)