Not 'outside' exactly, but I usually put Beo into his enclosure to eat bones - while leaving it open. I can't have stinky bones all over my bed.
The x-pen is his 'safe zone' so if he steps outside of that safe zone he gets his bone taken away, teeth brushed, hygiene stuff, etc. He's learned that if he doesn't stay in that one little area his bone will be taken away. Now, this doesn't mean I can't go in there or that he's allowed to be food possessive…I make sure to go in there touch the bone while he's eating, etc....but I can't take it away from him, or force him out. He has to be outside the safe zone in order for me to do that. For awhile he would keep bringing it out, but they catch on quickly when it comes to food.
In theory you could do the same for outside.
But, if you haven't already worked on food possessiveness beforehand maybe not the best idea....dog has to be secure with you taking away things from them first. Clicker training is great for things like this; then later maybe connect it to a command. It has to be consistent in order for them to really get it.
Otherwise, the easiest thing to do is put up a physical barrier to keep them from going where you don't want them to go.