I think the sourdough factor may be important. I don't know that there's anything magical about rye flour as opposed to other flours, but I can attest that it doesn't taste at all similar when rye is mixed with other flours, or when the leavening agent is a cultured yeast as opposed to sourdough starter.
Thanks for the feedback on oatmeal, ratgirldjh! I'll make sure to strain the oatmeal goo before putting it in my hair. The technique I use now with flaxseed is to strain it before allowing it to cool; it loses some of the mucilage but it allows the mixture to rinse clean out of your hair. (Learned it on the LiveJournal no' poo community, courtesy of westcoastred.)