There is a way you could do it - I'm thinking of it right now - but unless you are up-front about it to your customers this could be classed as theft (effectively installing a virus on there computer), and anyway they would soon find out and then the s**t would hit the fan.
They are totally fine with it, cos they are my irl friends.
What idea you are thinking about?
Is there any options exactly like dev fee? I was thinking about 2 .bats and windows task manager with delayed start, but i think that there will be missing much H/s.