Rules only enforced by honesty have certainly been considered previously. For example players not looking at (and listening to) things that should be hidden from them in Widelands was discussed back in 2003-2004. It is trivial to modify such a game to see and hear everything. In just an evening, someone familiar with C++ but not with the 0 A.D. code can check out SVN, figure out which dependencies are needed and install them, get it all built, look around the code and find the places that need to be modified, change and test until it works. (Some things in the game might be hidden from the user by code written in JavaScript and would need some extra effort to modify.)
The linked wiki page discusses how this could be solved by a client-server architecture, like FreeCiv has, and how much bandwidth might be needed. An idea not mentioned there is a hybrid approach, where a set of trusted servers run the simulation in parallel like 0 A.D. does now, and each can act as a server for clients nearby on the network. That could for example keep intercontinental network traffic low, while using more bandwidth on the shorter routes to the users, enforcing incomplete knowledge of the game state.
Another approach that was not considered back in those years, was streaming the game (video and audio to the client, input back to the server). That would use immense amounts of bandwidth, which is actually available in many places nowadays, while enforcing incomplete knowledge and preventing many kinds of client-side game automation (like autotrain or logic to make units do something useful when they become idle), which some might consider to be cheats.