Thanks for offering this, but I'm not keen on expanding our support for S3 specific options, at least without some significant proven need/demand, whereas this seems to be desired for a one-off scenario specific context.

If it helps, you can override our config files using our logical theme system, so you could effectively apply the same config value to your instance at runtime using the follow functions.php theme file:

<?php

use BookStack\Facades\Theme;
use BookStack\Theming\ThemeEvents;

Theme::listen(ThemeEvents::APP_BOOT, function () {
    config()->set('filesystems.disks.s3.token', 'mytokenvaluehere');
});