@Shaos got boards !
He also shared some informations he dug, such as https://github.com/hshutan/45x7-flipdot-controller: we learn more details about flip voltage (10-15V) so it's right in my range of +/-12V (though there is one more diode drop, which could be reduced with a Schottky).
I'm also curious about the following remark : ".... there is little risk of a flip dot being energized for more than a few hundred microseconds"
I don't know the cycle time of my machine but I was anxious that the pulse would be too short. I thought a dot would need a pulse that would last something like 100ms but Harrison implies that it's in fact shorter. Given that the RES15 relays have a propagation/latency of around 3.5ms, it's actually a good margin yet not excessively long... We'll see !
Maybe it's a coincidence but hackaday.com has just published a story about modern flip dots screens, the expensive kind...
Oh and have a look at the gorgeous dots at https://github.com/hshutan/45x7-flipdot-controller