I'm not so sure the lichens will survive, but what I've done (for pieces that actually fit in my oven, which admittedly, hasn't been many) is to spray them wet
with the diluted chlorhexidine* (or similar product, I've used others too, like F10 & Nolvasan*) that I clean cages with, then bake them @ 250* for an hour or so.
The reason for the spray is to kill pathogens left by birds & other wildlife...things that might not die off in the oven. Never had any negative issues that I know
of from using such branches, and the bigger branches don't even get baked...only on my hot sunny driveway, but they tend to get re-sprayed a couple times.
(also use a wire brush to remove any loose bark first)(*and just don't use any diluted bleach solution for this as the fumes will never come out of porous wood,
& they'll be toxic to your animals)
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