Making it impossible to adventure is indeed extreme, especially if one is in the relay browser specifically to see the flavor text. The biggest issue I can think of with a warning is that it could get annoying if it comes up every adventure in the relay browser for somebody wanting the flavor text, so if warnings occurred in the relay browser perhaps resolution options should be "Bail out", "Adventure anyway (once)", "Adventure anyway (this session)". Alternately such warnings could be controlled (and disabled) by an option.
Regarding warnings being worth it, it would eliminate this surprising behavior for this case, and seems like it should be fairly straightforward. Checking for the sources that can alter the text seems like a couple of lists, and then just hooking that into the before-adventure stuff. Granted, I don't know the code so it's probably at least 10 times as hard. But it certainly would be more tractable than parsing all the flavor text possibilities. (This is compounded by some ambiguous text substitution. I don't even know how I'd approach the problem, which is why I jumped to warnings in the request.)