I have recently planted up a fernery in my garden and have put in a few old logs for character. A larger log / stump I chiseled in some holes to plant with something such as a small fern, an orchid perhaps? An reconmendation for what would grow well in such a space. It's shaded by an oak tree too.
Almost all plants you choose for these holes will need drainage, unless you choose water or “bog” plants, which will instead need constant supply of water.
I’d highly recommend drilling drainage holes.
For the holes chiselled into the log:
For the surface of the log:
While there are exceptions, generally the plants listed “For the surface...” prefer to grow on things... they are epiphytic (grow on other plants) or lithophytic (grow on rocks). Placing them in a hole will do them no good and may in fact bring about their early demise.
It’s important to recognise that epiphytic and lithophytic plants gain their moisture and nutrients from the water and air and accumulated organic material nearby. This is done in conjunction with fungi and bacteria. While you don’t need to grow mushrooms on your logs, for success I would encourage an environment that supports mycorrhizal fungi.
Your plants will take about 12-18 months to truly establish. During this time you’ll need to be diligent with consistent (but not excessive) watering and during the warmer months and while the plants are actively growing, half strength liquid feed combined with half strength seaweed emulsion.