Variable spawn timer? No. Although someone with a bit of coding knowledge could create something to begin casting/cancelling during the expected spawn time. They would also have more accurate timers than someone manually trying to time. Worth doing? Probably not but then again you're talking about people smart enough to skirt around the 3rd party app restrictions. It is baffling how smart some idiots can be... also a bit sad that an average Joe can find resources to install and use something they really have no clue about (not saying that's you, just anyone in general that wants to cheat... often it's one smart guy helping a bunch of idiots).
Surely, this would never happen! Remove the keys already, so we can start arguing over what's in the zone (including /picks)!