There appears to have been no particular custom against them.
When the Star-K came out strongly arguing that quinoa was not kitniyos, I'd heard that there was talk of then certifying other semi-grains about which there was no custom, such as Job's tears (or teff, for that matter). Instead there was strong opposition to quinoa, so the Star-K stopped there. At least that's the way I heard it.
That doesn't mean it's not kitniyos; it means that no major US kashrus organization has publicly gone out and called it "kosher l'Pesach for Ashkenazim."
Some have interpreted kitniyos as a categorical ban that would include plants unbeknownst to our Ashkenazic ancestors if they meet certain criteria; if so, one could argue that newly-identified semi-grains are included.
Rabbi Moshe Feinstein zt'l wrote in Igros Moshe OC3:63 that there was no categorical ban; what makes something kitniyos? Only if a custom developed about it. He also suggested that they stopped adding things to the blacklist or else there would be nothing left to eat! One of Rav Moshe's proofs is -- why for 500 years was mustard prohibited and caraway allowed; which of those two would you more logically expect to be cross-contaminated or confused with wheat?
The plain reading of Rav Moshe's responsum (addressing peanuts, actually) would indicate that Job's tears should be permitted.
Those reading kitniyos as a categorical ban would disagree; some even claimed that Rav Moshe would agree quinoa is kitniyos because it is so absolutely, clearly a grain ... therefore some would prohibit.
Lastly, this is all a moot point if we can't identify a production of these seeds that we are confident were not cross-contaminated with wheat or barley. On that question, you'd have to consult someone who knows the food industry well.