What about Carfentanil? According to what I've read on the subject (which I admit isn't much) it's 100 times more powerful than Fentanyl, making it 10k more powerful than morphine. As an aside, 10,000 is the exact quantity of a 'Myriad' which is why I ended up reading about this drug; it's a myriad more powerful (misuse of the noun myriad admittedly) than morphine, but I digress.
I don't know much about naloxone admittedly; I do know it's used on heroin users who OD but that's about all I know in terms of its efficacy. I also don't know how long a human could live with (say) 150 micrograms of carfentanil in their system, but my guess is the enemy would be dead before trying to reach for their epi-pens.
Of course, if this drug scales in this manner and still has room to grow, it's entirely possible someone can build a new synthetic analog of carfentanil that's 100 times more potent AGAIN.
The bigger concern to my mind is biodegradability of the drug. Dump this on a populace, you may well wipe them out as well as any wildlife that goes to feed on the bodies, but if it has a half life of (say) a year, the entire area could well be a no-go zone for decades. Also, as we learned from Chernobyl, wind conditions could well spread the love further than you like.
As an idea it sort of works, but there are certainly limitations to how you would deploy it that need to be resolved, as well as how you protect people on your side from accidental exposure.