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Examining existing OTFs, I cannot find a decent definition of the aalt feature.
If you dissect an OTF, it seems all features supported by it are concatenated into a single large list (perhaps two; although the 2nd usually looks suspiciously like the first). Should one reconstruct the aalt table by looking up every possible 'complete' list in the rest of the features? Even then, the MinionPro.fea sample from Adobe (revision 2.012) shows some additional lookups added to the aalt table.
Perhaps this was never meant to be re-engineered :-)