Hi mate, thank you for your words.
Indeed, Alpha tiles don't work on reforged. So instead of that, as Feelsgoodman says, you can use ruined ground.
1- in object editor tab, find ruined ground. It is on cliff/terrain doodads
2- Chose any ruined ground. There are many, but they are grouped into 2x2, 4x2 and 4x4, and make sure you set "all" option on tileset settings.
3- For selected ruined ground , set model path route to .MDL (just replace the entire route to .mdl)
4- On terrain edition, choose destructibles/doodads palette, then cliff/terrain -> ruined ground2x2 (the last one you edited)
Put the ruined ground anywhere on the map, the terrain will disappear.
Optionally
Use grid (press g) and path map (press p) for better terrain control when adding transparent ruined ground
That's all
Tell me if it works for you.
Greetings