J. Karl,
Firstly, unless there is something I am not seeing, this is not relief carving, but rather, incise carving. Secondly, I do believe it very easily could have appeared on an early Lancaster style rifle. Thirdly, why not practice it even if you don't use it? It can only make you a better carver. It is simple, which means it is easy to practice, and it will help you get your line depth practiced.
Since this is your first gun, don't try for anything to elaborate. The simplicity of this design should be easily achievable with a little practice. In fact, a beginner can achieve pretty good results if they practice just a little bit. When I started with relief carving, I practiced my first design several times before applying it to the rifle. But this is incise carving, which is much more quickly mastered.
Another thing... I wouldn't worry about crossing stylistic lines so much. I have looked at many, many different schools of embellishment, and none seem to follow anything specific, with the exception of patch box design of a couple schools. Now individual carvers did have their own styles. Like J.P. Beck outlined his relief carving deeply, whereas most other makers didn't, but not in all cases did he do this. Carving, like engraving, was probably just personal style, and really doesn't cross stylistic lines really. It was either Baroque, or more likely, Rococo. So unless you are making a bench copy of a specific gun, I focus more on the carving that was common to the period of time which the rifle comes from.
Matt