Me. I've made the switch from Ag to airlines twice. The money in Ag and the lack of money at the airlines always made me go back.
The regional that I worked for had restrictions on outside commercial flying and I suspect most of them are the same. I'm also a real Eastern pilot. My time was short, but I think they allowed it as long as it didn't cause a legality issue.
Most regionals are like sweat shops and they will want all of your time available to them. My experience was they were not willing to work with me at all with outside flying except when the staffing would allow for a leave of absence, but if you own your own business you might pull it off if you employ someone that's willing to step up and cover for you.
Or just wing it and hope you don't get caught. I wouldn't know anything about that