In case anybody else is looking for an answer to this, here’s how you do it.
Install this plugin: https://wordpress.org/plugins/widgets-on-pages/ Using its settings, create a new “sidebar”.
In Appearances -> Widgets, put the Subscribe2 widget into the “sidebar” you created. Set it to forward subscribers to the page of your choice.
Now, on your Subscriptions page, use the Widgets-on-Pages shortcode to embed the “sidebar” with the Subscribe2 widget into the page.
You’ll need to leave the standard Subscribe2 shortcode somewhere on the page for the subscriptions to work. Avoid redundancy by giving it div tags styled to “display:none”.
