Further to this macro, I thought I'd try the same thing using qBittorrent, as it's more ubiquitous than Peerflix, and as such may be of more use to more people.
The idea:
With your mouse hovering over a magnet link, trigger this macro. Assuming it is your browser's default application for torrents, it will open the magnet link in qBittorrent, wait for the file to reach a sufficient size and then open it in VLC. I'm using the same shell script suggested by @Sleepy to pause for file size, and as far as I can tell, I haven't made any glaring mistakes, but at the moment it just waits and waits.
Any help appreciated.
Edit: just had to increase the minimum size limit. Updated the macro below.
Play Magnet Link (qBittorrent).kmmacros (48 KB)
You may need to re-capture some button coordinates and you will need to set your own download folder. The actions that require attention are marked out in green.