If the goal is to remove bad execs, then a document trail can help, although I'd suggest starting with some statistics like "over the last 3 months, we moved the goalpost 8 times, which led to an effective throughput of 4 weeks of work being done rather than the expected 12 weeks. How do you think we could improve these conditions?" Collaboration first.

Keeping email threads for reference is probably plenty data enough, btw; "signatures" sounds like the wrong approach. Maybe even just summarize the direction given in a wiki document with a change log with time stamps and requesting person, which you can review once in a while, and the sheer length of it might be enough to bring the point across.