I never played marathon, but I remember searching for stuff about the halo marathon link on google. On one of the forums I read that bungie said that the halo and marathon universe do NOT intertwine. Though I also read that some ex bungie guy said they were the same incarnation and yada yada yada.
I'd believe bungie over an ex bungie guy. So I'm guessing they are not related.
I personally think that that planet thing is in fact Onyx, remember that there are a lot of unanswered questions that pertain to Onyx...
The planet style resembles that of halo anyways, so it is possible on a graphical note. That blue shine that comes over the planet could be one of those huge combines sentinal constructs...think about it.
I am the pain, pain in the ass.