"It looks like they favor the TAC/Invensys system. I'm thinking it's a web-based ethernet protocol communicating over the college intranet (IT network) and maybe using an IT network server (I don't know if that's a good thing). Their specs is calling it a LONWorks open protocol. But researching the links sysint gave me, it looks like most Invensys controllers and devices are BACnet. I'm going to be asking them some questions tommorrow but it looks like I'm in for a rough ride."
Any of the options including your Siemens system can route over Ethernet. Don't let that confuse you at all. At some point you will still have a field bus to the individual controllers. Using the Ethernet can be advantageous and sometimes not. For instance, a Lonworks fieldbus does an excellent job of moving data controller to controller making a reliable network. Putting Ethernet between critical controllers wouldn't be as smart. However, routing to a front end or another subsystem is not so bad.
I really think if you state you want LNS Lonworks that's your best bet at getting them to play fair with you. Most vendors can deal with that IF they HAVE to, including Siemens. That way if this thing tanks you still have maximum options for recovery.
Tell Ameresco you want them to follow the Army Corps Lonworks spec or Lonmark.org spec as a MINIMUM standard and work from there.... (see how that goes over)