For anyone trying to do the same in future, here's what I did, in detail:
I defined a global variable named scenefrommqtt (leave it empty, so it's settable)
I'm publishing an MQTT message under reactor/mqtt/Expr/scenefrommqtt/set, with value "mykey" (the quotes are important)
On Reactor's side, I'm using the variable as a trigger, using the same name defined for the Rule Set
It's working very well for me, and it's easy to setup. Thanks @toggledbits for both MSR and for pushing me to think of current features, instead of asking for new complicated ones 🙂
Now I have just one single vera code running, and I'll migrate it later, leaving the Vera just as a glorified ZWave bridge until it'll work.