I ran into the same issue you are having right now about a year ago when I first setup my 55G tank. While I don't know what the tree itself is I was given a lot off good advice about getting a general ID
First off what mainly grows in the area you found it in? Or in this case what grows up stream from where you found it?
Was it water logged when you found it? It seems to be dry now, but if its been in the water for a long time it may have already leached out the chemicals in the wood.
You seem to be leaning towards cedar from what I've read...when you cut it did it smell? This is how I got proof positive that the piece I had was cedar. I cut into it and got that nice earthy cedar smell.
Now then...if you cut into it and smell cedar...well there's good and there's bad cedar (I researched this way to long). From what I got the cedars that you find in the northern parts of the country should not be used for aquariums. There are aquarium safe cedars though. These mainly grow in the south in swampy and marsh areas in the South East. I can't really break it down anymore there, but from Michigan it's probably not safe.
Blah...and I just read that people smelled cedar on it...so ya...it's probably not safe as is...so I would say if your willing to wait soak it. Soak it for a long time not just till it becomes water logged, but do water changes every few days over the course of a few months. Otherwise the epoxy idea would be a good one. Just make sure to use an epoxy that is safe in water and cures inert.
So...I think that's everything that I remeber from my driftwood ordeal...if I missed something please excuse me...it's late here.