FYI - Magpul has a few options - maybe not full length as your definiton- the XTM Panel rail covers I use not only look great and are extremely modular, but they make outstanding grips for your hands. I could not imagine my 15-22 without them and honestly, I love my quad rail with 3 panel-sides and bottom AFG, I am starting to think that a full four-sided quad rail with XTM panels is maybe the second best there is- I might take the MVG off my 5.56 AR and go with a quad rail and XTMs, its thst great for your hands.
Oh and you can of couse use Magpul's ladder rails- and if u want, ladder rails on 2 sides, XTMs on top and bottom, vice versa etc.