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[This is an old thread, but since search digs up old threads, it's worthwhile to have a correct answer.]
"...all my photos have the same aperature and shutter speed, but they differ in their ISO. ... I think the adjustment in EV units would go something like this: exposure_adjustment = -log2(ISO/baseISO)."
LR's Photo > Develop Settings > Match Total Exposures command makes precisely this adjustment -- given a selection of photos, it adjusts the Exposure slider so that the effective exposure value of all the photos matches that of the most-selected photo. If the photos' original exposures differed only by ISO, then the adjustment to the Exposure slider will be given by the expression - log2 (ISO / most-selected ISO).
As an example, I set my Sony RX100 V to "M" and Auto ISO and took four photos (ordered here by increasing exposure value):
Then I selected those photos, with the last (the one with the highest EV) as the most-selected, and did Match Total Exposures:
The first three photos got darker, as expected.
This table shows that LR adjusted the Exposure slider for each photo by - log2 (ISO / 2000):
"Exposure 2012" is LR's internal name for the Exposure slider in later process versions. (You can use Any Filter's Sort command to extract these fields into a spreadsheet, if you want to experiment for yourself.)
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