Sounds like a fun idea.
Your first impression of me may be that I am a dedicated Nurgle player, which is not far off from the truth. But before you start throwing tomatoes and telling me to get lost, the CSM army I have just started working on is not dedicated to Nurgle. Well, unless you count 2 units of Plague Marines, but it is my interpretation from this thread that a little cult troops don't hurt. They're mainly just there for gameplay and to add theme.
Which legion this is should be evident from looking at my current avatar.
It infuriates me when GW creates a totally kick-ass color scheme and just leaves it at that, with little to no backround info on the newly conceptualized army. But alas, they can, will, and have. Thus it is up to us players to take an interesting concept and craft it into reality.
My interpretation of the recent addition to the 'Eavy Metal's Chaos Space Marines section, The Cleaved, is that they are an army not fighting for Nurgle or Chaos in general, but for themselves. My ideal background for them is that they used to be a fairly standard Space Marine chapter, until their planet was struck by a deadly plague (yes, Nurgle is involved) and was infected. Thanks to their superior structure and immune system (in comparison to regular humans), they were not wiped out immediately, but rather, their bodies have begun to horribly decay and melt, skin and bone turning to green/brown pus and oozing out of joints and vents, staining their bone white power armor. Much of the chapter has already died from this; their bodies now a pile of noxious goo. (stored in the chapter's Vindicator, interestingly enough) The remaining marines have forsaken their now dead chapter planet and taken to the stars in a space hulk, desperately searching for answers and a cure.
They are not strictly loyal to anyone except themselves. Their old chapter doctrine encouraged them to hold a strong belief in brotherhood and looking out for one another. With this notion implanted firmly in their rotting brains, they will stop at nothing to save their chapter, even if it means turning on their old allies and destroying entire worlds to do so...
As their is no official fluff for The Cleaved, this is what I had to come up with. I view them as a Renegade group due to the fact that their decisions have become affected by Chaos and as they are no longer completely loyal to the Emperor. They are certainly no longer a Chapter, and haven't become a Traitor Legion (yet). And as out of 10 units of guys, only 2-3 are loyal to Nurgle, I also view them as currently undivided. They don't work or fight for Nurgle; in fact, if anything, he is considered to be their worst enemy. And they have realized that there best chance at survival may be to join with their great enemy... Quite a conundrum.