Can Power Over Ethernet be daisy chained?

Generally no, because in switched Ethernet, all links are point-to-point. There should be one power sourcing equipment (PSE) device (e.g., an injector) per powered device (PD). It would be up to the switching fabric in the switch (in your case the Ethernet switch in the WAP) to support this.

An exception is the relatively new Cisco PoE passthrough [PDF] range of devices which:

can draw power from an upstream (PoE+/PoE-capable) switch or a router in the wiring closet, to power itself and to drive power downstream to the IP devices connected to it.

Another option is a HP IntelliJack Gigabit switch, which can power up to two devices, but only if supplied by a 802.3at-2009 (PoE+) source (or a local DC supply, as described in the manual PDF).

Apparently it's possible. There's a manufacturer of surveillance cameras that offers daisy-chained PoE cameras. I don't know to what extent their solution follows the standards, I assume they have added some proprietary "hacks" to make it all work.

VIVOTEK is pleased to launch a revolutionary series of daisy-chain PoE network cameras, IB8367-R, IB8367-RT and IB8338-HR. The three uniquely-designed network cameras allow for both PoE input and output as well as direct connection with other PoE network cameras without an additional power source, thereby facilitating long-distance surveillance system implementation in factory areas, parking lots, crossroads, and outdoor sidewalks.

http://www.vivotek.com/vivotek-embedded-poe-extender-design-r-series-network-camera-revolutionary-invention-accelerates-the-evolution-of-outdoor-and-long-distance-surveillance-applications/