The _p.s._ strikes me as a bad idea given the way BT works*, but a proper event hook should do the trick:
> Let's see an example of how arguments are passed to command:
>
>
> $ cat hook.sh
> #!/bin/sh
> echo "Called with [$1] [$2] [$3]"
> $ aria2c --on-download-complete hook.sh
> Called with [1] [1] [/path/to/file.iso]
>
The `--seed-time` and `--seed-ratio` options kind of automate many use-cases, too.
( _*_ better check your settings and read up on BitTorrent)