"Does NFPA specifically require a pneumatic solution or is a powered timer adequate?"
NFPA does not specify either type. Alarmingly, nor does it specify a delay interval the lock must be unlocked! Yikes! (see Deadly Pushbuttons for why this is a problem) It does specify a visibility requirement, which may bias selection toward an electronic type:
NFPA 101 – 2009:
7.2.1.5.9 A latch or other fastening device on a door leaf shall be provided with a releasing device that has an obvious method of operation and that is readily operated under all lighting conditions.
7.2.1.5.9.1 The releasing mechanism for any latch, other than for existing installations, shall be located not less than 34 in. (865 mm), and not more than 48 in. (1220 mm), above the finished floor.
7.2.1.5.9.2 The releasing mechanism shall open the door leaf with not more than one releasing operation, unless otherwise specified in 7.2.1.5.9.3 and 7.2.1.5.9.4.
7.2.1.5.9.3* Egress door assemblies from individual living units and guest rooms of residential occupancies shall be permitted to be provided with devices, including automatic latching devices, that require not more than one additional releasing operation, provided that such device is operable from the inside without the use of a key or tool and is mounted at a height not exceeding 48 in. (1220 mm) above the finished floor.
7.2.1.5.9.4 Existing security devices permitted by 7.2.1.5.9.3 shall be permitted to have two additional releasing operations.
In my opinion, the most useful piece of code comes from IBC on the issue of RTE.
IBC part 1008.1.4.4 (2012):
"A manual unlocking device (push button) shall result in direct interruption of power to the lock – independent of the access control system electronics. When the push button is actuated, the doors must remain unlocked for 30 seconds minimum. The push button must include signage stating “Push to Exit” and must be located 40” to 48” vertically above the floor and within 5’ of the doors. Ready access must be provided to the push button."
Most jurisdictions in the US adopt IBC in addition to NFPA 101, so the '30 second' interval is the standard. However, neither code specifies 'pneumatic' or 'electric' RTE type.