No, I wouldn't.
I'm quite certain no entity comparable to a deity exists within our universe (with reservations and wiggle room), and so have no need of reaffirming that.
Aside from that, the stone wouldn't prove the [non]existence of any entity named "God" to anyone but myself, as only I would have
a priori knowledge of the stone's validity, thus rendering it useless as a tool to inform the masses. You specifically stated that
I have
a priori knowledge, and so my answer is based on that single stipulation: only I know of the stone, and of it's validity.
BTW- which "God" are we referring to?
There is no way I could know a priori that a stone saying "god does (or doesn't) exist" is valid. There is no logic to validate it.
The question is hypothetical, and thus doesn't
require a justification. It operates outside the realm of our reality, and thus certain allowances must be made. Obviously, sandy vagrants don't know of the existence of random pebbles with scribbling on them (which are, in fact, buried in the middle of some desert), nor would they somehow be able to completely verify the validity of this pebble's claim without ever even viewing it.
But that's our world. A hypothetical question doesn't obey those rules, and is posed in such a way as to be obviously hypothetical (usually).
Where is the imagination?
And hey! This is the
PHILOSOPHY AND SPIRITUALITY forum. Of COURSE they are going to be hypothetical situations posed!