Viability of well decayed logs for Hugelculture

Would very decayed logs(think cellulose) be worthwhile using for Hugelculture or would their nutritional value be too diminished? They would still hold water well I imagine. I have some firewood logs left on ground
in woods  for several years. (I think the beetles have had their way with them and move on to greener pastures)!
Also: is lichen covered bark beneficial(esp regarding toxicity)?
Bill in Claysville-(no really; stone filled clay w bedrock shelf 13 inches down! I've been told that the clay can be up to 80' deep here in CNY glacier country(NYS Finger Lakes)

I definitely don't think it would hurt. I imagine it would hold water even better than a fresh log. You'll get a finished mound quicker though so that's one thing to consider. I would probably use some of that and some fresh stuff if I had it. If the decayed stuff was all I had then I would probably still go for it. Keep us posted on how it goes!

You are describing the ideal wood to use for buiding hugel mounds.
The more "rotten" the wood is the better it will hold water.
Fresh cut wood is not suitable for this, it will not soak up enough water and is the primary reason so many hugels fail the builders.
The wood in a hugel is not there for providing nutrition to plants as much as it is there to hold water over the long term so the plants have it available in times of draught.
The primary nutrient supplier is the fill that you put around the wood core.

By the way, when wood rots it is the cellulose that rots away first, the lignin is what gives a tree its structure.

Lichen is a bacteria that is in symbiosis with a fungus, these organisms are not usually toxic.
Lichen is one of the primary rock breaking organisms in nature, producing some of the first soil found in rocky areas.

Redhawk