As I understand it the major problem is that engineers need a lot of cash to get even more cash if they exit the company and need it in 90 days. Possibly very naive question...couldn't you include a clause along the following line in your funding round(s): "Investor X guarantees that they will offer to cover the necessary cost to exercise all options in return for Y%". So basically if engineer A leaves and could exercise options the investor must offer a "bridge loan" with guaranteed ROI (either cash or just keep the equivalent options).

Good investors shouldn't mind this clause that much (I'd think) as it takes away one major point of worry for engineers and lets them concentrate on their job from day 0.