If braised meat is cooked at 200 degrees in a perfectly sealed pouch in the oven, does the temperature rise above boiling?
Any increased steam pressure would raise the boiling point, not lower it. However, I doubt that you will be able to seal the pouch well enough to withstand any serious pressure.

What may cause boiling though, is the fact that ovens cycle on and off, especially at low temperatures. The amplitude of  the cycles depend on how good your oven is. At 200°F, your pouch will probably experience above-boiling ambient temperatures during the on-part of the cycles. Of course, that doesn't mean that the contents of the pouch is above-boiling, but there may be.

However, I don't think that the braising liquid boiling slightly now and again is going to be a big problem. Sometimes when I've braised meat, there's been some boiling, and the results have still both looked and tasted great.