I wrote a python script acting as a MQTT Client using the standard MySensors Serial Gateway:

https://github.com/arendst/mqtt-mysensors

It is based on the MQTT Ethernet gateway as provided with the MySensors package. It supports the same message structure as
<topic_prefix>/<nodeid>/<childid>/<variable_type> [<value>]

Depending of the presence of <value> a C_SET or C_REQ message is send to the gateway. It recognizes ON and OFF for V_LIGHT. So publishing 'mysensors/0/0/V_LIGHT ON' will turn the light on for node 0 and child 0.

Before use please update the user configuration values according to your environment at the beginning of the script.