Ok, I said I'd report back and here I am. BLUF, it doesn't work on my Downtube 8HN. I tried above and below the middle QR, and the way the seat sits you can't get the seat forward enough to engage the pin that keeps the seat on the stinger. Depending on how your stem is adjusted it may work for you, but I'd seriously doubt it.
I am pretty dissappointed since I really like the IBert. It's a great seat, but it just doesn't seem to work with the dimensions of the downtubes.
It might be worth asking Yan if he thinks it would be safe to support that much weight on it anyway. But I imagine his answer would be know.
I took some pictures, but I didn't think they were worth putting up. Let me know if you'd like to see them however, and I'll oblige.
Sorry it took so long, sick daughter and pregnant wife. Busy night.