Is it possible to set a fixed Peer ID for every transfer (in "settings" menu)?
I know it can be set manually but it is very inconvenient.
A few torrent forums have "semi-private" trackers,which allow DHT but a fixed ID and special tracker address are needed for "privileged" internal seeds.

I support this too.

You can do it using advanced tracker options: https://support.tixati.com/individual%20tracker%20options

For example this:
sometrackerurl.com
header:%0D%0AUser-Agent: rtorrent/0.9.2/0.13.2
peerid:-lt0D20-
xfer_peerid:-lt0D20-
should change (both peer and tracker) peer id only for torrents with sometrackerurl.com

Peer id is 20 character long, if you'd like to have fixed peer id just fill remaining characters after -lt0D20-