The Myo will be tough to mod.
Instead of screws to hold the headlamp together, Petzl uses the meathod where the put a plastic post though a hold, then mushroom the end.
So, you can't really take them apart, then put them back together. You can get them apart with some cutting I am sure (be sure to clean the knife and dress all wounds so as not to get infections when you cut yourself), but it will be up to your epoxying skills to re-assemble.
So, in order to put a pouch battery you will have to use the existing wire and connect it external to the headlamp. If you don't mind a wad of heat shrink or electrical tape, sure you can do it.
Another option would be to solder a plug inline. This would give you the flexibilty of plugging the original battery pack in, or another. A phono (RCA) plug would work pretty well.
You know that Petzl makes a Myo 5 with a belt pouch. If you can afford it, it would be better than doing it yourself. You could even sell the old one on ebay if you needed to raise funds.
