I am the same as you riding regular on a board and naturally left foot forward on my mtb. I am not sure if it correlates to your snowboard riding style though, the majority of people I have ridden with mtb left foot forward regardless of which way they snowboard. You might be over-thinking it but now you have me doing it as well. Can bet I will subconsciously note it for the people I ride and bike with.
Just like riding switch, it is something that can be learned on a mtb as well. On very long rides that include plenty of descending, people will tend to tire one leg faster since it supports more weight. I had to force myself to learn to ride switch on the bike so that I could conserve energy and strength but it takes time and conscious thought to work at it.