Since 0.2 or 0.3mm is only 2 or 3 100-micron layers worth of resin thick, most Formlabs resins will probably cure pretty well that way. It should work even better if your mold is clear, but if you’re really making a part that thin, it shouldn’t matter too much. Some people have done a roughly similar thing, where they print in one color, and fill shallow impressions in the part with resin of a different color, and then cure that liquid resin with a Form Cure or other UV source. The effect is sort of like an inlay.
I’d suggest starting with the recommended post-curing temperature and time for black resin, and if that doesn’t seem to work, double the time. Because of the light-absorbing black pigment in it, Black resin does take more energy to cure than a lot of other resins.