Just found another thread which solves this issue, but has unintended side effect: Filename for shortcodes when using custom RSS output format
Basically, if I change this (default):
[outputs]
home = ["HTML", "RSS"]
page = ["HTML"]
to this:
home = ["HTML", "RSS"]
page = ["HTML", "RSS"]
then my quote.rss.xml shortcode get picked up for rendering content inside RSS and it works as expected.
Unfortunately, this solution has one side effect - it generates additional per-page rss files which I do not want. Also each html post gets a link <link rel="alternate" type="application/rss+xml" ... to this RSS feed, which can confuse an RSS reader.
I think this is a bug - if RSS format is enabled for the home page, then when I render page Content in the rss.xml it should be considered to be part of the home page and inherit its format.