@ Tom - Over here while walnut trees grow in certain parts (they dont fruit) the damp cold or humid when warm is not good and the longevity suffers as rot win the battle. However this is easily remedied when mothernature has determined its fate and us creative types get stuck in.

i have left bark on, its deep crevices serve well and look good. My waterdragon enclosure (rainforest setup) is still furnished with walnut branches though steadily declines obviously, since 2005-6 due to high humidity. My bearded dragon enclosure (dessert set up) branches still as new ( well still as when tree surgeon wapped his chainsaw through it!) - no decline.

with the assessment of your own grow area conditions aside, there was no signs of toxicity to plants as roots clinged to wood. My orchid walnut solo mounts have been remounted to larger pieces due to growth and mount deterioation (am using specialist tropical aquarium wood/pieces now). It holds water well but constant moisture reduces life compared to cork bark. The mycelium of whatever fungi native by you/ growing by you (unless urs a hot/dry climate) will be sure to spring to life in cultivation. I would imagine spreading the spawn of oyster mushrooms etc (& other edibles) would look fantastic.

If anything I would conclude that any epiphyte species of plants that like a drier less constant moisture situ like oncids would suit ideally when using walnut as mounts. Wood/bark looks great. Many bromeliads would suit as their root ball can be side lined and the funnel used for its care. The moisture holding properties of walnut not good for tillandsia (air plants) or epiphyllums ( these are very vulnerable to pathogens and would pick up infection in a bottle of bleach). Moss not required as pad for mounting as walnut retains moisture under bark in its cork/ tissues (depending on grow situ ofcourse).

I have always picked up and used/recycled loads of organic medias from wild or friends gardens. It is only the disinfection/ sterilization process that i have reassessed and improved avoiding chemicals. I adore those wind storm tree breakages that are covered in lichens and mosses.