I have used half-logs that have worked out well, I just put the log against the side of the cage, then tape newspaper to the outside of the cage over where the log is so it makes the half log seem completely closed off with one side open. You have to watch out for mold and weird stuff on them though, that was always a big problem when I would use them for my tarantulas with high humidity requirements.

And like other people already have said, just clean the cage out well and PAM it, it should kill whatever the little bugs are. I've had random bugs show up in my tarantula cages before as well, after I would clean the cages I never saw them again. If they show up again then I would consider switching substrates to see if that changes anything, even if the bark is heat treated that won't stop bugs from making it their home after you place it in the BPs cage.