As far as I can tell ash wood is safe, but the seeds might not be, but chins shouldn't eat seeds anyway. As far as preparing a 3 ft long branch, unless you have a very large oven then I don't see a safe way of preparing it. Wood needs to be cleaned and at least baked, some people also boil before baking too. You need a way to heat it up hot enough to kill any thing that might be living inside the wood, so either boiling or baking.
Rather then making one perch I'd cut the branch into pieces and have 3 one foot perches, or 6 six inch ones or however you want to cut it up. That way they are small enough to fit in the oven to bake. Also I'm sure the chin would like having multiple perches more then just having a single one.
Another thing to remember is the wood used needs to have never been sprayed with chemicals. Trees trap the chemicals in the wood for a very long time, some decades to forever, depending on the chemicals used. Even in small amounts (too small to work to repel/kill bugs anymore) it can make a chin sick or die.