I have not. I have to much in corny kegs for me to change now.

 

So you intend to take a finished, uncarbed beer and crumble cookies or pop-tarts into it to provide priming sugar? Doesn't seem like a good plan unless you want a gooey, murky mess. Stick to basic sugars for carbing and play with odd, starchy fermentables in the mash where they'll be processed properly by enzymatic action.

 

^^^^^ Second. Gunk - solid residue of fermentables - has no place in bottles or kegs. Besides, there are no enzymes in beer to convert the starches to sugars. You get haze, gunk, and very little carbonation (unless you use American breakfast cereals in which case, there's no reason not to use pure, refined sugars).

 

Using cereal or pop tart in a cask could give You some interesting flavors but your carb level will be hard to calculate. Probably be under or over carved.

 

Literally dunking a cookie in flat beer...Not my idea of a good time.

 

I'll take your word for it.

 

I've casked two beers so far. A brown ale and a kentucky common. Both came out great. I used 5L mini kegs. The first time I only used half as much priming sugar (pure cane sugar in my case) as I would for that volume if I were bottling. The 2nd time I threw cause to the wind and gave it a full dose. I left it to condition inside a 5 gallon bucket as a safety precaution, though it wasn't needed. The 2nd attempt, the american brown ale, was awesome. Folks that normally turned their noses up to anything darker than Sam Adams were surprised how smooth it was and didn't have those coffee flavors. I just brewed a Marzen today. I might put 5L of that into a mini keg for a Cask Conditioned Oktoberfest.

 

Since it's obvious that you'll do it no matter what advice you might get, maybe go for a proof of concept on a small scale before you commit an entire batch. You could get a bottle-conditioned beer that will have some active yeast (most Belgians are good candidates), let it go flat and carefully pour it into a (preferably) plastic jar on top of a crumbled cookie or two. Let it sit for a couple of weeks and see if carbs up and turns into something you'd want to drink.
Keep in mind that Pop-Tarts, cereals and most store-bought cookies have preservatives in them that will likely inhibit microbial action. Also bear in mind that most baked goods will be made with some sort of butter or oil and even small amounts of fats or lipids will kill any sort of head retention.
Have "fun"...

 

Point is that those off-the-wall adjuncts are used in the mash and not post-fermentation. There are a lot of ways to use interesting ingredients, but you'd do well to figure out what you can put into a cask and have the beer remain viable.
I would 100 percent be interested in an oatmeal-raisin milk stout, but I might go about it using all the ingredients of the cookies that would give me the flavors I want without adding baked cookies that might be made with butter and kill the head. I might mash with lots of toasted oats, Honey Malt for lingering sweetness and Crystal 60 and Crystal 120 for dark caramel/dark-fruit flavor. Add brown sugar and raisins in the boil and use plenty of lactose, vanilla and a hint of cinnamon in secondary. Milk and cookies in a glass of beer!!
Hell, I've made it sound so good, I've talked myself into it...gonna have to try it!