Sorry it took me so long to get back to you, I fell into a Fallout New Vegas-shaped hole for a while.
It's definitely possible to convert this into use for a vertical shmup, but it's not within the scope of the project at this time. Since using the UI Interactable mod for a HUD is very popular in the GB Studio community, I targeted specifically horizontal shooters since they'd have the most screen space for most users.
THAT SAID, here's a secret: you can totally do this with the engine as it is right now, you just have to get a little outside the box. When you format your player sprite for vertical shmup sections, have the right-facing sprite frames rendered as facing up, and make sure the player's facing is set to the right. Then you can simply have the fire button mapped to fire upwards. The only limitation here is that the firing arc stuff isn't designed to work with a vertical shmup.
Here's an example sprite I whipped up for what I mean (you're free to use it, if you like):
Since this method won't allow you to flip the sprite to face down on the fly without changing the player sprite, there's no reason to use the left two frames for the ship. It's a good place to put the death explosion animation for your ship, for instance.
