Building Unity as a DLL for another application

Hi All - I'm new to Unity3D, and looking to generate a 3D environment as part of an existing product. My product is built currently for .NET under Windows but the non-windows-specific parts of the product will build under Mono for use under Linux. Most of the functionality of the product is built as DLLs and set up to be Plugins to my main application. Such plugins perform data processing or generate visual output in 2D such as data analysis graphs, etc. I am looking to generate 3D visual output of my data using Unity to provide the interactive 3D environment. From everything I have read in the manual it appears Unity is geared to generate as an EXE rather than as a plugin to another application? Is there a way I can do this? If so, what versions of .NET could I build my application on and still get this to work?

Thanks in advance for any advice :) Tim