Werkt een MQTT Broker-server op de NAS540

Theoretically yes. The NAS has plenty of CPU and RAM. (And storage, I suppose). In practise, if you can find a statically linked binary for Linux Arm5 or Arm7, it will probably work. If it's some Python package it won't. (Python on the box is outdated, and you can't add any libraries. No pip).

A non-statical linked binary might work, depending on it's dependencies.