Will 802.3af device work on 24v passive poe?
The standard leaves that open, basically.

The regular negotiation process will fail for a compliant device, at which point the device either falls back on some builtin support for dumb injectors, or disables PoE completely, in which case it is expected to not draw any power. It may optionally retry after some time.

Most devices use a lower internal voltage anyway, so they have a step-down converter, most likely behind a set of protection diodes or a full rectifier. As long as the voltage minus the voltage drop in the cable and the forward voltage of the diodes is still enough for the regulator to work at a duty cycle within its design specifications, the device can accept the power. As the voltage drop in the cable is likely to be significant, there is usually quite some headroom.

The other question is whether the device will accept a dumb injector that doesn't speak the protocol. That is fully device-dependent, and may be different between different revisions of the same device model. The only guarantee you can get is from the device's datasheet.

That said, I'd go for a standards-compliant injector or a manageable switch with (equally standards-compliant) PoE ports for any serious installation, simply because the cautious startup sequence will protect you if broken devices are connected.