As I've pointed out earlier, a lot of customization would have to go into it, but making a side scrolling action RPG on RM isn't outright impossible.
For a better picture of what's required, RM was designed to make top down 2D RPGs where characters move in four directions (up down left right) and battles are handled by default in separate scenes in a turn-based fashion. A side scroller moves just backwards and forwards, and the possibility of climbing up or more importantly, falling down. Battles are on-scene in real time.
From this assessment, you can already figure out what needs to be done:
- Defining gravity for falling objects
- Optional, defining additional physics for other movement like gradual acceleration while running, jumping
- The battle system
- An animation handler for all the extra animations needed to show different actions
- Actually drawing the animations themselves, from the hero to every single enemy character
- A heads up display of player stats like HP and MP outside of the menu
- Weapon/item behaviors, particularly for fancy stuff like boomerangs, guns and bombs
Touhou Wandering Souls was made on VX. Like I've said, there was a script that had the basic mechanics (basic movement, jumping and player physics) for making a side scroller shared sometime ago. It wasn't as polished as Touhou's, but it's a start. I don't recall one being made for Ace, but if you could find the old one on VX it's just a matter of tweaking things for compatibility.