I'm thinking that Lelouch probably doesn't want to fuck with the consequences of a blanket "do whatever I say forever" Geass, *especially* after what happened with Euphie. Possible brain damage, them possibly having to have him nearby in order to do anything, even trivial actions like eating or sleeping (yeah, that's not going to arouse suspicion,) and of course the fact that it's possible the Geass wouldn't work at all. Not to mention the fallout if the Geass was removed for some reason.
It sounds like the easy solution, but there's too much that could go wrong given that he's never been able to really figure out everything regarding how his Geass works.
The Cornelia Geass is close, but the trick to it is that since it's not a direct 'do whatever I say' Geass but rather a command to recognize him as someone who he's loyal to he's still capable of independent action- with a "do whatever I say" Geass he'd be a mindless drone and Lelouch would have to order him to eat, sleep, take a shit, etc etc, with the Cornelia Geass he's not a mindless drone but he'll still follow his every command, for the most part.
Which is, of course, why he couldn't have done it with anybody else, really, there aren't too many other characters who are as loyal to someone else as Guilford is to Cornelia.