Can I make just the filling? I’d like to top things with it.

I think you could follow steps 1 & 2 as written. For step 3, I would sauté the apple slices with several tablespoons of butter (in batches if necessary) in a large heavy sauté pan over low-medium low heat. When they start to soften, sprinkle with the sugar and the flour and stir/sauté until thickened; you may have to add a few tablespoons of water during this process to prevent burning and facilitate the thickening action of the flour. When the apples are tender and still warm, stir in the caramel mixture. You may have to play with it to refine your version. Good luck, let us know how it turns out!

My guess is that it can be done. After all, cobblers are basically pies with the crust on top. There are also upside down pies like the classic Tarte Tatin.

Rather than do this in a metal pie tin, I'd use a borosilicate (a.k.a. Pyrex) or ceramic baking dish. You could start by covering it with a sheet of aluminum foil with some incisions cut to vent out excess steam which will help give it a more solid end consistency.

Just bake until the apple slices end up to your desired consistency. If it's still too moist toward doneness, I'd remove the foil to hasten evaporation. Since there's no crust, you don't have to worry about that being under or over baked.

Anyhow those are my thoughts on how I might go about doing this.

Best of luck.