I've tried using lemon juice in melomel, it doesn't work. Lemon juice is far too acidic & will do quite a number on your yeast. Also, it adds NO lemon flavour to mead. To get lemon flavour, I've learned (the hard way) that fresh lemon zest is the best way to go. Spend the $4.99 on a decent zester, it's quick & easy to use & definately worth the price. You can make a sort of syrup with the zest by simmering it in a bit of water (also kills undesireables) with sugar added. Or you can use it fresh if you want. How much to use is entirely up to you. I've only used lemon zest to augment other flavours, so I've never used more than the zest of 1 lemon per gallon, but there's nothing I know of that says you can't use more. The actual zest will fall to the bottom with the rest of the lees eventually. Hope you find this info useful. Regards, GF.
EDIT: You might try one of Keller's lemon wine recipes: http://winemaking.jackkeller.net/reques48.asp
No idea if they're any good or not, if you try one, let us know how it turns out. Regards, GF.