i haven't seen one as far as i can remember, but i can see the finished conversion in my head...
should be fairly straight forward...
i would use his Termie armour as the base...
the arms are perfect, the legs iconic, and the head is a seperate piece...
you could even use the front half of the torso now that he's finecast (should be easy enough to saw the Termie torso in half)...
my biggest beef with any of the truescale kits on the market are the straight greaves...
i like the flared shape of the legs on a Marine, and Marneus is the king pimp of the 40th millenium...
he should be rocking some bellbottom greaves!!!
not to mention, the Termie legs are blinged out like a mofo

...
finally, you will be able to use him in
GW stores and events, as he is made of
GW parts...
win/win

...
sounds like a fun project...
best of luck...
cheers
jah