Create an entry in your userSetup.mel script and source cm_renderPresets.mel from there so that on startup a render presets menu is created. This menu is populated whenever it is viewed so it should catch new presets as they are created.
For those who may have never created a userSetup.mel file this a simple MEL script you need to build for yourself using a text editor. Create a new document and save it to your maya scripts folder (eg. myDocuments/maya/2009/scripts/) with the name "userSetup.mel" and put the first entry in it:
Save the file and make certain the cm_renderPresets.mel file is in your scripts folder as well so that they are both sourced and run when Maya is next launched. It's that easy.
I've added a workaround hack to handle mentalRay presets and a new menu item to use for saving presets (this fixes the mentalRay side of saving render presets). I could only find two points of failure and I think I have them addressed. Hopefully someone can take a moment to test it and report back any findings. If anyone tests this under earlier versions of Maya could you please let me know how it works out? Thank you!