i'm not sure if anyone really knows what pasteurizing does to the flavors. has anyone really done a side-by-side taste test?? dunno, interested to hear if so
if you add in a lot of sugar (ie more than the 2-4 gravity points bottle conditioning usually consumes) in the form of juice concentrate and/or sucrose you are going to risk having bottle bombs as long as the yeast is still active. if you are unsure about this then the plastic bottle will certainly help tell you if/when you are getting into a dangerous level of pressure. once you are in the 'gusher zone', where a well chilled (1 or 2 days in the fridge) bottle gushes out when opened, they are overcarbed and you probably don't want to heat pasteurize them or they could blow.
i advocate the 'hard but not rock hard' policy where when the plastic bottle still has a tiny bit of give to it then they are just right. this pretty much always works for me, and the few times i have pasteurized bottles using this as a guide they have been fine. so you could always get them to that level and then pasteurize just to be safe.
the other thing worth mentioning is that you might have very low yeast activity in the bottles, since you have high alc %, so they may go very slowly, but they may keep on going very slowly past optimal carb and into the gusher zone. i don't know if that will happen or not- it depends on yeast, alcohol %, nutrients, temp, and the amount of sugar you add back! good luck