Only stumbled onto this thread now, very interesting read.
I've been taking Chinese skullcap powder, around 5 g every night, for over 18 months to improve my sleep.
I had read the potential liver damage info when I first looked into it but I have had tests every few weeks/months for this entire period, including during a 3 month treatment with valcyte and my liver has been fine the entire time. I do take a couple of grams of NAC daily though and have been taking milk thistle for a few months now.
Unfortunately the skullcap doesn't seem to affect my PEM in any way. I might try take some during the day in the future and see if I notice an effect then, but taking it each night hasn't done anything aside from very mildly helping with my sleep.