I would agree with the other posts. Pressure treated wood is like it's name. It is injected with chemicals at high pressure to the point of refusal. The wood is super saturated and literally takes years for it to dry out. And in the drying process it twists and turns and darn near walks away from you. I always wear full protection when working with this nasty stuff. This wood will most likely move during carving as you release the internal pressure by removing wood. Since the wood will be above ground I would question the need for treated in the first place. A good rot resistance wood such as cedar, redwood or white oak when properly painted will last for decades. My mail box has a treated post in the ground and then the rest of the post above ground is designed larger to slip over the treated portion. It has been in place now for 10 years and not a hint of rot. The snow plow has taken it off twice and one drunk driver but no rot
I like the idea of alternative materials and the concept of aged ivory sounds intriguing. My only caution would be to have high contrast especially at night if it's function is for emergency location and addressing.