Guys, I don't have a Halo costume done yet (it's on my To-Do list), but I've got years' worth of experience with hot costumes - I've worn a movie-grade Darth Vader suit since 2003 as a member of the 501st Legion. Based on how they go together and what they're made of, respectively, I'd compare a Mjolnir suit (depending on what kind of undersuit you use) with a TK suit in the 501st - that's a stormtrooper, for those not in the 501st. My Vader is DEFINITELY hotter than my TK, although even the TK can get steamy. My recommendations:
Concur 1000% with those who've recommended a helmet fan. I'd go so far as to call it a necessity, although more for its utility in keeping the visor clear than anything else. Still, it can be REALLY nice having a fan blowing cooler outside air into your face, if you can rig it up that way. If you can rig others in the chest or back armor, that might be helpful, as well, but that will depend again on what kind of undersuit you're wearing.
Concur 1000%, again, with calls to hydrate. Do not worry (not TOO much, anyways) about going to the bathroom. You'll sweat more out than gets processed into urine - I never need to go to the bathroom in my Vader, no matter how much I drink. If you need to go, maybe it's time to get out of costume anyways.
If you're highly heat-sensitive, strongly consider a cooling vest. I really like the second one linked earlier in the thread, with the integrated pump and backpack reservoir - that's going on my research list, I think. I have a vest marketed to costumed sports mascots, which has pockets around the whole vest for long ice packs which I can transport in the insulated bag they came in or in a freezer chest I bought several years ago (it has an actual refrigerator/freezer unit built into it, and can re-freeze expended ice packs overnight at a multi-day con like DragonCon - it cost like $700, though, I bought it when I worked at the power plant and made more than I do now). With the ice vest, I'm good for about 3 hours in nice weather or indoors, 2 hours or so of parade walking or in really hot weather, before the ice packs are melted and at body temp. I have 3 sets of inserts, so I can suit down, reload the vest, and suit up again if I need to.
Ice vest:
http://www.dura-kold.com/dkkvest.htm
Freezer chest - can't remember the brand name off hand and I'm at work at the moment. Mine looks a lot like this one, though:
http://www.freezerlink.com/scout_portafreeze.htm It's quite heavy, but I can plug it in during the multi-hour car trip down to DragonCon, and in the room when I'm there, making sure that I can wear my Vader and Robot Chicken Vader costumes. It's worth every bit of the effort to move it, and it's more than paid for itself IMO.
Do NOT mess around with heat exhaustion or heat stroke - it's no fun at all. I did the DragonCon parade as Vader a few years ago - 08, maybe? - and nearly passed out after the parade, before I could get back to my room and suit down, despite having my ice vest on. Having to stand around in the park before the parade started didn't help (took time off the ice vest's duration), and they don't call it "Hotlanta" for nothing. Thank God for concerned passersby who got me water in the lobby, and who made sure I got a spot in the elevator as quickly as possible.
