Military flap holster?...I say MAYBE...only due the possible orientation of muzzle as you draw it. Is this a rearward-canted (angled)holster? and if so, can it be adjusted to hang muzzle vertical? Whatever your plans are, I suggest demonstratign for your posse/Ros/Match director PRIOR to shooting.
With MY holsters(homemade slim jims), definitely a YES ....Been demonstrating my flavor of the reverse or "twist" draw for a number of years in both NCOWS and SASS matches. Early in the game, I was asked how I was going to draw my pistols (considering both are swung the "wrong" way)....so I demonstrated to match directors, posse members and ROs how I was planning to draw before my first stage. First in "really" slow speed, then at my normal (slow) speed.
Note that all twist or reverse draw techniques are equal. Mine preferences follow. First, the holster must be oriented straight down. The pistol must be drawn close to straight as possible, clearing the holster, then swung downrange (straight away from me) while oriented ups-down....once it is clearly downrange I complete the twist. Properly executed, this is as safe as any other draw method (properly executed). Some twist the pistol while in the holster, but I do not do this because I find it easier to clear leather with the long-barrels, riding at waist height, while fully reversed. If twisting in the holster, I notice more tendency to dip the muzzle rearward during the draw.
My preference is to use this style with my 1860 percussion Colts, as I believe it is a historically correct mode of carry (easy to sit stand or move) while packing a long barreled pistol at my waist. I draw my right pistol with right hand (twist draw), but I'll often draw my left holstered pistol with my right hand, using the crossdraw movement to avoid transferring to my strong-side.....unless I'm going double-duelest or gunfighter (there is - or used to be a SASS rule against this)...What do I like least about firing gunfighter C&B (?) - BAD caps.
If I move up & out of the percussion era, I prefer traditional barrel-forward (rather than butt-forward) carry....just seems "right" to me
Slim