You'll probably get many replies saying the front stall would be the most appropriate location. One way to find out for sure, is by trying the loading in both places. You won't have to go far to discover if the rear most stall unloads the tongue weight too much.

One thing you didn't mention is what type of trailer you purchased, a GN or BP. A BP will take some experimenting, if it's a GN, you should have no problem, especially with a dressing room.

If the trailer tracks straight, in and out of a corner at highway speeds, and exhibits no swaying, you should be good to go. With a horse in the rear stall of your BP, do you still have a reasonable amount of tongue weight? If you don't, don't try this exercise.

Depending on the make and model of your trailer, various manufacturers place their axles at different placings under the frames. Few trailers will have exactly the same configurations and tongue weights, so comparing an xyz trailer to an abc is not accurate. The length of the dressing room and its loading will change the tongue weights. The further aft your axles are located, and the longer the dressing room, the more likely hood that a single horse can be placed in the rear most stall.

I would think that with a 950lb horse, your tack (most tack trunks I have seen weigh a ton, hay, feed etc will probably end up weighing about the same.  Not to mention the hassle of having to unload everything to get to the horse.

As Gard states (no I'm not translating), the only way to know is to weigh it, but I dont think it will make much difference if you have a substantial tow vehicle.

Measure the height of the coupler while hooked up empty. Then measure it with the horse loaded in the rear stall. If the measurement is higher with the horse loaded, I wouldn't even try....it means you have taken weight off of the coupler. It should drop a little, but could very easily stay the same.

Is this the trailer? Looks to me as if the horse would be standing right over the rear axle. I would think you would be ok, but would follow Tresvoltes advice and measure the hitch loaded with all your gear and the horse, and unloaded.

Taking a look at the Calico, it is just like every other 2H BP with it's axles basically fully rearward.  At full rearward the pivot point is still several feet forward (between the axles) with the horse on a slant at the very least his butt will be behind the pivot.  I can tell you with my current trailer, and any other stock/slant I've fooled with, loading a horse in the rear stall will most certainly unload the tongue weight.  I use a Sherline tongue scale, the tongue weight went from 650 empty to 550 with the horse in the rear.  Despite still being in the acceptable BP tongue weight range of 10-15% the trailer was considerably less stable and I doubt the outcome of any 'situation' would be good. 

I would not make a habit of towing that way even IF you perceived the handling of the trailer to be acceptable.  First, basic trailer loading/design principals dictate 60% of the load forward of the axles center, unless you're packing a lot of stuff there is no way you will be able to achieve that loading the horse in the rear stall.  Second, since you had to pose this question I will have to assume you are not that experienced towing, as such during "testing" you may miss the subtle tell tale signs of instability.  Not trying to be rude, just trying to get you where you're going.

I didn't interpret your post as rude at all, though I do hope your comment about being inexperienced isn't a slam.  It is true that I am but I am doing everything i can to learn the right way to do things in order to be safe. 

At any rate, thanks to everyone for the advice and comments.