LoginWithSteam API with app id parameter?

I'm not sure how feasible this is, but it would be very helpful for me (and some others from what I've searched) to be able to do Steam login (on one title) with multiple Steam app IDs. This is mainly to accommodate a Steam game that has a demo product.

A while back I made a post about Steam demos and since then I had rigged up a system using loginWithCustomID and steam-cloud to allow my demo players to use the same title as full-game players. Occasionally there are issues though and it would be a huge relief to be able to just use Steam login for all cases.

The reason I think this is possible (and could be an optional parameter) is that when we grab our web API key from Steam, that key is associated with a group. In our case (and most cases I would assume), the group includes the full game and the demo. But unfortunately in the Playfab add-ons dashboard, we can only specific one Steam app ID. It would be cool if we could pass a steam app ID into the client login API call and it would override the one in the dashboard.

Alternatively, being able to specify a second Steam app ID for demos would be great too... the only issue I imagine in this case is that the Playfab side of things wouldn't know which app ID to use unless the login API specified the app ID.