To start, the reasons why they might lead off against righties and lefties respectably are obvious. Mclouth has the speed to lead off, but he bats .230 against lefties. Diaz has an obp against lefties over .400 but against righties he bats .255. The two are also arguably our best base runners. That being said, getting them on base ahead of the heart of the order is to our extreme advantage. It is also worth noting that against righties and lefties respectably they have a comparable power boost. Thus if it were possible to platoon them at the lead off spot, it would result in a lot more base runners coming from the first batter. It would also seem to enhance the consistency of what the next batters are looking to do. 

Also, since Mclouth's BA is .230 against lefties, and Diaz's is .255 against righties, it seems essential to keep them out of the leadoff spot against pitchers on the same side of the plate. Seriously, they should not be batting first or second with those batting averages no matter how fast they are. However, they are still our two best base runners, so we want to maximize their obp percentage against pitchers they struggle against, and get them on base ahead of the gauntlet we call a lineup. To do this I would propose Diaz and Mclouth consistently alternate between 1st and 8th - which should be easy to maintain even with Diaz' platoon since he will always be in against lefties. It's also worth mentioning that the 8 spot is good for Melky when he starts for Diaz both because he's a comparable baserunner, and because everyone batting in the 1-7 spots has more power or a better reason to be in the middle.

Batting in front of the pitcher at 8th their ability to draw a walk will go up against the pitchers they hit less. In this scenario Mclouth is in position to get on, steal second, and get bunted to third when we're facing lefties. Diaz and Melky have some ability to pull this off as well, and all 3 can score from 2nd on a base hit. This will help them be a more consistent base threat by helping them get on base more on days they might otherwise struggle. It also keeps our best base runners ahead of the order even when they aren't leading off (w/o paying for their lagging obp on alternating days). I should also mention that in this scenario our leadoff hitter is capable of producing runs like a heart of the lineup guy (Mclouth an .833 ops against rhp, Diaz over 1.0 against lhp). This strategy will thus maximize both their run producing ability (by getting diaz on and over for mclouth and visa versa) and their ability to get things started.

This platoon would not only maximize production from our one and eight spots, it would also enhance their consistency for the sake of the rest of the lineup. What's more, it's also advantageous to the two players' psyches since the role of getting on and working the bases is consistent between the two spots.

so to lay it out:

Mclouth/Diaz

Prado

Chipper

Glaus

McCann

Escobar

Heyward

Diaz/Mclouth

Pitcher/pinch hitter

I should also note that the pinch hitter will almost always be Diaz when facing a left handed reliever in a game started by a righty, putting him in the same role.