This is a good question, and the answer is more complicated than one might expect. There are already two mechanisms where a client’s (or station’s) transmit power can be limited, but they were developed for purposes other than co-channel interference and the brief answer is that we don’t find them generally reliable in limiting clients’ transmit power. 802.11d allows the access point to advertise the regulatory or country limit, and 802.11h introduced a power constraint element where a local transmit power limit could be applied, as an offset to the regulatory limit. But as you suggest, 802.11h only applies to DFS-compliant devices in the 5GHz band and by no means all 802.11 devices are 802.11h-capable.
If I may add a plug for the most exciting Wi-Fi event of 2012, a solution is approaching in the shape of the 802.11k amendment and the new Wi-Fi Alliance 'Voice-Enterprise' certification. 802.11k introduced concepts specifically aimed at limiting co-channel interference and saving battery life in handheld clients by providing information that allows clients to reduce transmit power. Firstly, an access point can advertise a transmit power constraint lower than the regulatory limit in the same way as 802.11h but in both the 2.4GHz and 5GHz bands. Secondly, 802.11k transmit power control and link reports allow a client to discover how its signal is heard by the access point, including a link margin figure, so it can reduce transmit power levels if appropriate. These mechanisms will be included in the Wi-Fi Alliance ‘Voice-Enterprise’ certification, look for the announcement later this year. It will take a while before the majority of 802.11 clients are Voice-Enterprise compliant, but we expect smartphone vendors to be early adopters and have high hopes that the Voice-Enterprise certification will gain wide acceptance.