In reasturants, they often have Butter, sauces etc on the griddle....in space without a lit burner undernieth. This allows the Pan to leverage the heat from the other parts of the griddle top. Mostly stainless steel pans...and sometimes will also sit on a low rack. Point is, heating something or keeping something warm is a standard practice for a griddle continuously on.  My guess however, the makeup of vessel you place on it might be impacted by high heat. I would suggest a cast iron stainless or other molecular makeup that is single material (not a teflan coated pan for example), would be fine even at high heat. I ain't no scientist mine ya, but seams to be a common sense response (in other words no warrantee implied or excplicit.) Ha....I would also consider the surface type of your griddle. 

As you now know using a pan on a flattop works ok. However, it doesn't work so well if you want to set a pot with a couple inches of oil on top to deep fry. The Blackstone just doesn't have the horsepower to get the oil back up to temp quickly enough (for me anyway).

I guess if one is in no hurry it might work out but it was too slow for me so I won't be doing that again - I'll continue using the portable induction burner.

 Sure you can use a pan on a griddle surface. I have used SS containers for melted butter or warming syrup on breakfast cooks. And have had a pot of hot dog chili at the ready when I did dogs for a charity car wash event. A flat bottom is necessary for good heat transfer. Temp control is not great. The chili I had to keep moving from the surface to the corner rails to keep it from boiling/burning.

My buddy Randy who cooks the gumbo at Brisket Camp had done gumbo on a griddle many times.  He used to cater at KCBS.  And he made the trophies in his wood shop, but not using the griddle.

it would be no different than cooking on a French top ( minus the temperature gradations) or induction burner. Yes, you'll likely have to clean the bottom of the pan more often if you also cook directly on the griddle top. 

By the way, I cook this way but over copper plates on my stove all the time.

There is a U tube chef, Robert Roy who cooks with a pan on the BS while he grills the meat and stuff right next to it. He makes up the sauce etc and then drops the grilddled meat and veggies into it.