Patched inner tube helped by pressure against tire?
It's not necessary for the tube to have the external pressure of being installed in a tire. If patched correctly the patch will hold just fine if there are days or months before it is used.

My normal habit is to travel with a spare tube. When I flat, I switch the tube out, and carry the old one home where I patch it later. In my garage I have a box of patched tubes that I reuse as spares and the patches always hold.

If you wait a few minutes after patching, you can, as you ask, fill it with air outside of a tire, and the patch should still hold. In fact doing that and then dunking the tube in a basin of water can be a good way to test that the patch was installed correctly before you deflate it and store it for later use.