Incidentally, it reminds me of the “acrobatic mystic” option from the Rules Cyclopedia; in case you aren’t familiar with it, a character of the mystic class can choose (at character creation only) to have some special acrobatic skills. To balance this out, the mystic must take something like a -20% experience penalty.
I’m not, in fact, familiar with the “acrobatic mystic” from the RC. In fact, I don’t think I’ve ever opened up an RC.
My first thought when I decided to eschew 2e (or later editions) and return to the old school, before I learned there was an Old School Renaissance going on, I wondered if going the Rules Cyclopedia route might not be the way to go for a simpler approach to the game. Seeing the prices on eBay and recalling the storybook style of the 1983 Basic set, I quickly abandoned this idea and soon thereafter discovered Swords & Wizardry and Labyrinth Lord.
Regardless of whether the “sub-class as an add-on” method has been done before or not, I think it’s a natural fit for thieves. Especially when trying to mimic the pulp fantasy style of swords and sorcery. If you haven’t checked out my take on the White Box Thief, go do so and let me know what you think.
It’s currently listed as a “2nd Draft” because I’m hoping to collect some more feedback, play the thing in a game or two, and ruminate a bit more before deciding on a final design. Also, the next issue of Knockspell magazine will contain a number of articles and proposals for S&W thieves, so I’d like to check them out before finalizing.