Can I have the U87 please. Hehe. That's one of my favorite mics.
Oh yeah! One of my faves as well - wish I could afford one...
It's a common practice to run the mics at a distance from the talent as you have seen in numerous "making of" clips. However, those rooms are constructed specifically for ADR and VO purposes; there is lots of "air" but they are fairly dead sonically. You don't
have to do it that way. Get the mic(s) in closer. It takes a little more time in post to match the EQ and tweak the reverb to mimic the space of the location, but I would rather have that than the room sound which would sound very odd and artificial if the scene is in an outdoor setting, and is very hard to defeat or even to make match with indoor scenes.
You're saying to just build a box where the rifle mic can sit in?
It's a shotgun mic in sound-for-picture parlance.
The infamous "VO Box" may be a solution. For his home studio the legendary Don Lafontaine had a 2'x2' box lined on all five surfaces with about two or three inches of acoustic foam and he placed the mic just inside the box.
In my studio the corner with the video monitor is covered with 1 1/2 inches of Celotex with carpeting on top of that. (The carpeting is more for looks than sound.) There is almost no bounce from behind the mic and I just get the talent in close, 3" to 12". As a mixer I would rather have something very "big" and "dead" that I can "degrade" rather than trying to polish a turd. I just suck out the some lows and some highs, tweak the mids until I match the tone of the rest of the dialog and then use my trusty Altiverb to match the natural reflections. And yes, outdoors there is some reverb/reflection, it just isn't very audible. The "trick" is to get a 'verb that can be heard when the voice is soloed but not noticeable when placed in the mix.
A buddy of mine uses the "Cone of Silence" (a Get Smart reference); it's just a very large piece of Auralex in an ice cream cone shape attached to the boom arm of the mic stand and around the mic. He hangs up a few sound blankets behind the talent.
As you seem to have a bit of audio experience your biggest challenge is the mind-set; sound for picture is all about making everything sound "real." Ideally you use the same mic that was used on the set. If you are trying to match production sound dialog I would lean towards a shotgun for outdoors and the MKH50 for indoors, but that is a call you should make with your own ears. If the entire scene is to be ADRed the choice is completely up to you. What you should probably do is use some personal time after the studio is closed and do some experimentation. Put up all the mics in various locations and search for the "dead" spot. Maybe a few sound blankets, gobos or whatever you have available to deaden it further. As you said, you want to get it right. You have all of the tools and the skills, you just have to apply them differently.