How to prototype user customization of an environment with multiple windows and dockable panels?

We are tasked with prototyping several different interactive window management schemes. The context is a complex piece of B2B software that's usually used on multi-monitor setups with 3-10 screens.

Currently the system simply pops out top-level OS windows for everything and leaves window organization and layout completely to the user. The result is always messy, unless users spend hours on aligning everything pixel-perfect.

We would like to test several techniques to assist the user in this respect. Some call for letting windows snap to each other and screen borders, others suggest tabs, other Windows 8/10-style tiles on a dashboard, yet others would like one monolithic, but incredibly customable "Master Window" for all modules as found in Visual Studio or Eclipse. So there are several conflicting ideas going around and we simply want to test what works best for our users in a reasonably realistic setting.

It is important to have something interactive here, not just stills. Otherwise we will only produce a lot of opinionated interpretations but no solid observations of users working with the system.

While it's quite easy to get our hands on sample content to be displayed in some windows, I'm having an incredibly hard time to find anything that we could use as a canvas for this content. We could use Firefox or Chrome to test out ideas for allowing windows to be combined into tabs, however splitting windows, docking them next to or into another, etc. requires a lot more than either browsers or Windows' windows management will allow.

One example of the ideas we'd like to test, image courtesy of AvalonDock for WPF

Ideally we'd need something as flexible as Eclipse or Visual Studio with all the content replaced with our mock-ups, so we can observe users arranging their work environment with different window management features turned on and off.

Short of implementing such an application frame ourselves, is there a good way to use other tools to prototype it faster?