Can a rechargable dash cam battery be replaced with a dc-dc converter in-place?
If you are able to set your regulated supply to 4.2 volts, this will mimic the max recommended safe charge voltage of the camera's built in LiPO battery. Based on my experience with a few charge management ICs typically used in such applications, it is a safe bet that at this voltage there should be, at worst, only occasional VERY low current "standby charge" energy feeding back toward your supply. I believe it would be insignificant and   could safely be ignored. 

You'll probably want to place a capacitor in the circuit, maybe in the same physical place as the battery, and maybe 220uf (just a guestimate). This  should further help fool the camera's battery charge management circuit to "feel" like a real battery is there. I'd also recommend sticking with a simple analog regulator rather than any kind of buck regulator. Such a device, though more efficient, can add high frequency noise which could leak into the sensitive analog electronics in the camera.