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A novel idea. There are a couple of possible downsides.
1. A refracting surface only needs to be figured to a quarter of the accuracy of a relecting surface, so the solar filter, while it may be good enough as a refracting surface, may not be up to scratch as a reflecting test surface.
2. If the solar filter has slightly curved, but parallel surfaces, it may still work as a solar filter, but be no good as an autocollimator. I don't know how they make these things, so I'm not sure whether or not this is a likely scenario.
However, if you have a paraboloid of known accuracy, or better still a spherical mirror, you could check the accuracy of the SF. If it's OK, you've scored a cheap flat mirror.
Geoff
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