Can anyone recommend an open source ESB tool/runtime which is extensible to include or compatible with BPM (either BPEL or exec BPMN)?
Ideally I'm looking for something with proven scalability, and a good set of graphical support/maintenance interfaces.
I'm aware of a couple of things which do this in the commercial world but a but not too familiar with o/s offerings.
I would recommend WSO2 ESB. It is based on the Carbon platform which offers a wide range of OSGi based components. BPM is one of the features available in the platform and can be installed onto the ESB. BPM is available as a separate product as well, but if you simply need an ESB which has BPM capabilities, you can simply install the BPM feature onto the ESB
It has a fully-fledged management console & Eclipse based tooling.
See
Eclipse based development studio http://wso2.com/products/carbon-studio/
Repository containing extensible features http://dist.wso2.org/p2/carbon/releases/3.2.0/