There are a few problems I see here: 1) stock option grants are completely arbitrary and sometimes end up very wrong 2) it's hard to fix that in the future because you'll end up at a higher strike price 3) end up being expensive and tax inefficient to exercise
The closest I've seen to people who seem to get this and have sensible solutions are Andrew Mason at Detour (progressive equity) and Dustin Moskovitz at Asana (larger grants, but back loaded into years 4-6).
I have great respect for Adam D'Angelo at Quora for suggesting a solution to the problem, have known him in school he's certainly smarter than me on almost every axis of intelligence, but I think there are other potentially creative solutions that might be better (although I don't know tax compliance).
For example I think you could keep the status quo, but offer the option for employees to exchange their options for shares (white meat) at the time they can exercise. Example you have options for 100 shares at a strike price of $50, at the time you leave the shares are worth $100, instead of having to come up with $5000, you just get $50 shares free and clear. I think there's still a tax hit issue, but at least it's not doubled with paying for the shares.