I was recently learning Rambam's hilchot tefilah from Mishneh Torah. After outlining the five kinds of statutory recitiations of the Amidah, he states that just as an individual (but not the tzibur) could offer a voluntary korban, so too an individual (but not the community) can perform a voluntary recitation of the Amidah whenever so moved (though some kind of change to the matbeah should be made to distinguish it from a tefillah chovah).
I find this rather intriguing and my question is this: Are there any traditional sources which go into detail about tefilah n'dava as a practice? It seems like a good springboard for meditative prayer practices and I'm curious as to whether any luminaries have written on it.