There are no such things as maya shaders for renderman! Theres just slim telmplates and sl sources. True slim is the tool that come swithe rat but you can also get slim standalone. AND parsing it externaly with tcl isnt hard.
Also the same thing kindof applys to mr shaders tough the user may or may not have doenne maya templates for the shader in question. (is the shader is compiled for your platform versus your version of mentalray it will work on whatever software you are as long as the mr verison is the same). And sourcecode is offcourse fdoable.
BUT havving a MR subsection si not that bad a idea.