I think the round faced locks on Georgian fowlers and pistols was purely a stylistic choice to reduce the amount of flat surfaces on the gun - especially on pieces where they chose to use round barrels. When you look at English Georgian period guns (I should refine that to "early-mid" Georgian, ca: 1720s-60s, I guess, since George III lived up into the 1800s) there are very few, if any, truly flat surfaces on them. When you study how they shaped the moldings, sideplates, etc. it seems a deliberate attempt to make everything with a radius to keep the visual flow.
As Mike pointed out, we have no documented pre-1750 surviving Virginia rifles. According to research by folks like Wallace and Gary, we know that rifles constituted a relatively smaller proportion of the gunsmiths' output at Williamsburg. Keep in mind, "James River Basin" potentially covers a huge area of Virgnia - and based on surviving later pieces, there were significant differences between rifles made in the far upper reaches of the James and those made farther downstream, near the tidewater area. (Question for Gary - isn't the "Faber" rifle thought to have been made somewhere down near Richmond?)
Wallace makes distinction for those guns thought to be from the headwaters and forks of the James region, which is sort of south and west of, but not far from, the Shenandoah Valley. Germanic/ continental style locks as well as English style locks crop up on guns from the late 1700s in the Shenandoah Valley, which is not surprising based on the demographics of the settlers and trade routes in those areas. Of course, these areas were settled later than the tidewater. But, and this is just my opinion, I would not be surprised to see a Germanic style lock crop up on an early longrifle from the far upper James area, but I would expect to see mostly English style locks on guns made over in the tidewater area.
Guy