After thinking this over some, I'm gonna add in the ability to select a date and a time for the kill. If the submitted date/time would occur before another guild claimed it, the new claim will require a screen shot of the achievement as proof. I'd really like for kills to be updated immediately. If this is a problem for the guild leader, they should authorize others to be able to submit the kills.
I don't have time to develop this new feature at the moment, but I'll add it on my list to get done before the next EQ content release.