If you are still so worried about an unprimed flintlock going off, use a frizzen stall.
Now that would just be silly.
Closing the frizzen in the first place is an unnecessary step in the loading process that could lead to an accidental discharge. Why ad another unnecessary step by using a frizzen stall to protect against a possible danger caused by taking the first unnecessary step?
Just leave the dang frizzen open! Keep it simple and safe!
At any range or event where I shoot, loading with a closed frizzen would first get you a safety lesson and then an invitation to leave if you did it again or argued about it.