The discussion concerning programmable MAC in Body Area Networks (BANs) is due to the demand for simple and low-power sensor nodes. Additionally, the diverse applications in BANs require low-level modifications to support adaptive services or custom functions. In this work, we propose a novel scheme for programmable MAC, which requires a minor modification to the beacon frame of IEEE 802.15.6-2012. Specifically, our main contribution is the attachment of a command to the beacon, which is broadcasted at the beginning of each superframe by the hub. The hub requests an action, typically a modification of a MAC capability field, by the nodes with a metric which satisfies a constraint. Thus, one command at a time, the proposed scheme is applied with a negligible overhead. Two adaptive use cases, based on signal strength, are implemented to demonstrate this scheme. Firstly, the hub requests the nodes with high signal strength to enable relay support and secondly, the hub requests the nodes with low signal strength to set a sleeping pattern. In the first case packet delivery increases significantly, while in the second case each node saves an amount of energy.