A way to create a master text frame specifically for footnotes?

Interesting - I'd like to know why you need this function - maybe there's another way to achieve what you need. 

I used to do footnotes manually back in the day the footnotes couldn't span columns. 

I'd create a master text frame on the master page, and set it to auto size, height only, from the bottom, with a text wrap on the top.

Then as someone else said - use endnotes (a feature I also did not have back then) 

You can then thread the footnotes onto the master text frame and use page breaks to force the text to the next page so it matches the footnote reference.

It was a pain though.

I'd love to know why you need this - just out of curiousity.

Theere is no such function. You could use endnotes and place them in different frames. Position and size should not done via master, but via object style in this case. Normally on the master should never be a text frame for content, except primary text frames, but a primary text frame would not be helpful here.