It's not uncommon to have torrents sorted into folders based on labels on seedboxes. Forcing the label to be selected before it starts downloading would be nice to have.
I came here to request pretty much the same thing. I use qBittorrent and setting a Category on upload will properly set the download location without having to manually move it later.
Am I correct in that the code first uploads the torrent and then sets the label?

Because I am calling some code with rtorrent whenever a new torrent gets inserted, and at that time there is no label set yet, but when I check a moment later I do see it. I see multiple calls to plugins/rpc/rpc.php when adding a torrent.

If this is the case, can you set the label immediately when uploading? Chrome Remote Torrent Adder does that for example.