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Colin,
If the blank is homogeneous and properly annealed then cooling should not result in astigmatism, but you may see a slight change in its correction for spherical aberration.
Typically, however, the disruption to the final image will be overwhelmed by the effect of the convective plume at the boundary layer.
The assumption here is that the primary cell or secondary support mechanism (rtv?) do not impart stress on the optics as they cool.
Easy way to test is to rotate the primary in the cell and see what happens to the orientation of the astigmatism. If it also rotates, then I would bet London to a brick that the mirror blank has internal stress (for which there is no practicable cure). If it doesn't rotate, then the problem is with the secondary, or by virtue of the manner in which it is attached.
~c
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