Was looking for the same thing and found a way to do this, thanks to the
forum software docs. Replying to this post here so that it helps others too.
1) Subscribe to more than default 5 posts:
Add ";limit=n" to the URL.
Example: http://forum.arduino.cc/index.php?board=7.0&type=rss2;action=.xml;limit=20
2) Subscribe to multiple boards with a single feed:
Add ";boards=x,y,z;limit=n" to the URL.
x,y,z are the board IDs. These can be got from the board URLs on the
index page Note that it should be "
boards", not "board".
Example: http://forum.arduino.cc/index.php?boards=7.0,8.0&type=rss2;action=.xml;limit=20
I'll probably write a python script to fetch RSS feeds to all boards and generate a OPML file which can be imported into feedreader. Will keep posted on any progress.