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Jorge --
It would simplify reciprocity testing considerably if the test exposure sequence could be controlled by the aperture, but the necessary range is much too great. The aperture scale of a typical enlarging lens rarely covers more than 5 or 6 stops—a ratio of no more than about 1:64—and the desired test time range is 0.1 sec to 1000 secs, (1:10,000) which is roughly equivalent to 13.33 stops, or a log range of 4.0.
ND filters are expensive, but although they're very convenient for this sort of home tests, they probably aren't absolutely necessary. You might be able to get useful reciprocity data by contact printing a step tablet on film under the enlarger, controlling the light intensity by placing sheets of fogged film under the lens. You don't need a focused image, and you can read the densities of the fogged film sheets with your densitometer to calculate the approximate exposure ratios.
Ideally the fogged films should read 1.0, 2.0, 3.0, and 4.0, but that's asking a lot! You'll almost certainly have to use two film layers to reach 4.0—and, of course, any stray light in the darkroom will invalidate the test of at least the longer exposures.
If you try this I'll be interested to hear how it works! 
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