In capsman you can limit Tx power under /caps-man channel
... even if you currently have something there, you'll want to create a new one (possibly similar to the existing one), but with tx-power property set
add name=lowpower tx-power=15
and then use that setting in /caps-man configuration
property ... If you only want to reduce power for certain CAPs, you'll have to prepare separate configuration sets for default CAPs and the low power CAPs, then select specific configuration set in /caps-man provisioning
add action=create-dynamic-enabled master-configuration=lowpowerconfiguration radio-mac=E6:8D:8C:49:EE:4A
add action=create-dynamic-enabled master-configuration=defaultconfiguration
Beware that rules in provisioning section are matched in usual top-to-bottom order, so make sure the default (without radio-mac set) is the last one.