No. I've heard it mentioned but never seen it done.
The short version: I wouldn't allow it. If a player tried it as a casual venture (whether as a hedge mage, a healer for hire, a blacksmith, or whatever) they would find that the entrenched local interests cut them out of the business - only certified guild members can practice the trade. Becoming a certified guild member is possible, of course... by retiring the PC.
I disagree with your decision, for the same reason I support Bullgrit's "player opposed ear scam" story.
Since you said "casual venture" that to me defines it as a side plot between the main quest. Players should be encouraged to pursue personal goals, and as a casual venture, it's not like its taking serious time away.
If this was the PC refusing to do interesting adventure stuff for the sake of his full time job running a scam, then as Bullgrit's players decided, "this isn't fun for everyone"
In 2e, my retiring PC (who ran a kingdom) set up a manufacturing of Continual Light lamps, and other easy magic items. He created a merchant company to distibute. Which was really a front for his spy network. I never ran it to get rich, it was basically an idea to self-fund my spy network without needing to manage a budget for it.