Xi Yin explains how to calculate the partition sum of the monstrous pure 3D gravity in terms of gravitational instantons.
V.V. Kiselev and S.A. Timofeev discuss the cosmological constant seesaw mechanism. Their approach is exactly I meant: the mixing of the flat and AdS vacua occurs because of the analogous instantons that induce decay - except that the corresponding bubble in this case can't grow and only induces the mixing. They also discuss how this small vacuum energy may be generated in the context of gauge-mediated supersymmetry breaking.