Does anyone have any resources on voluntary prayer/tefillah n'dava?

Question

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.

Answer 1

I've come across it a few times in Shulchan Aruch Harav.

For example, in siman 582, it says that if during aseres y'mei teshuvah, you said "melech ohev tzedaka umishpat" instead of "hamelech hamishpat" and you waited more than the time it takes to say "sholom olecha rabi" or you stated the next berachah, then, some say, you need to go back to beginning of "hoshivo shoifteinu", and some say that, since "melech ohev..." mentions mishpat, you don't go back; so it says that what you do is after shemone esrai, you pray a nedava, although you don't add anything extra in it because you're saying it because of a doubt as to the halacha.

I know this may not be considered a full answer, but see here: http://meta.judaism.stackexchange.com/a/2155/6886