It's not uncommon to par-cook meat for barbecue, as in these recipes:
The meat is basically cooked before it goes on the grill. The grill is applied at the last minute to add extra flavor and to glaze the barbecue sauce.
There is considerable debate among barbecue aficionados about whether this is really appropriate. Some flavor is lost to the water (unless you use sous vide), and you lose the opportunity to spend hours in smoke. You certainly won't win any barbecue contests this way. But it's definitely one way to get falling-off-the-bone 'cue.
As for safety... the same 2/4 rule applies here as elsewhere. The food should spend no more than four hours in the 40-140F range before eating, and no more than two hours before storage.
The meat will be above that range while being simmered. As long as you cool it quickly (perhaps in a sous vide bag, in an icewater bath), you can put it in the refrigerator with minimal time in the danger zone. If you freeze it, defrost it in the refrigerator before cooking.
You can then let it spend perhaps three hours on the grill before it has spent too much time in the danger zone. Since it is fully cooked at this point, you won't need to spend more time than that.