There is no power setting for any Airport device. The output power is specified in the 802.11 standard. Variations from that are not only a violation of the protocol but also a violation of FCC regulations and potentially subject to fines running well into five figures.
Legally speaking, transmitters never have a minimum power, only a maximum. Although the power may not be editable via the GUI, it's quite likely to be available in the exported settings file. This file is in XML and allows changing of numerous things that are unavailable in the GUI. I've personally used it to override the airport's selection of an IP address for the station itself. I don't know offhand if power is there. If it is, it's probably in a percentage, and is currently set at either "1.0" or "100", and will not accept a value above its licensed maximum of whatever 100% represents.
I haven't reviewed the 802 standard recently, but iirc it does allow for lowering power for base stations, especially in congested areas, and also allows for clients to lower their transmit power when in closer proximity to the base station. This works to lower the effects of congestion when many transceivers are in close proximity. (cell phones definitely work this way)
