How is spread-spectrum implemented in low-duty cycle transmitters for 868/900Mhz, 2.4Ghz, etc
I am no expert in this - there will hopefully be much better answers soon. The following majors on one "lead" which may be highly useful or a complete blind alley - please advise. 

802.15.4 is probably a useful starting place for understanding. This is the physical and MAC layer that the higher level layers implemented by Zigbee and ISA100 and Wireless HART and Microchip's MiWi are based on. (All but one of those are Wikipedia references). 

802.15.4 is targeted especially at low power infrequent communications. Part of the low power in specific implementations is achieved by low transmit power levels, but the protocol is focused on achieving low power. One method of many is preassigned slot allocations.

This paper seems to have useful things to say Co-existence of IEEE802.15.4 at 2.4 GHz but I had trouble finding a downloadable form of it.This one can be read online. Equally annoying version here 

Microchip AN1204 - Microchip MiWi™ P2P Wireless Protocol - may even be useful :-).

This Real Time Automation tutorial is wandering around the general subject and may be apposite.

UMASS Lowell - Wireless sensors lecture interesting and possibly relevant. 

Microchip AN1066 MiWi stack

Not directly related information but relevant and interesting enough to be included