Would a tagine be better for high altitude braising?
The benefits of using a tagine will be marginal at best. It will save some moisture loss over a dutch oven, however the cooking time will continue to be around 25% longer than conventional recipes as the tagine won't raise the temperature over your lower boiling point at all.

Your moisture loss will continue to be high due to the increased cooking time, irrespective of the tagine's shape - and especially as at 2 quarts it will be quite small, with a short 'cooling tower.'

Colorado State Uni has some information on high altitude cooking which may be useful.
http://www.ext.colostate.edu/pubs/foodnut/p41.html#3k

As Ecnerwal &amp; BrownRedHawk note, a pressure cooker is a far better option. As a rule of thumb, for every 5 degrees above 100c that you cook food, the time it takes to cook is cut roughly in half. A decent PC is able to braise at 15 PSI, which is 248f. Even at 5000ft above sea level, with a modern, non-venting PC your food will lose no moisture and braise in no time.  There is no need to try and keep the PSI lower to simulate cooking at sea level; for braising, higher is better as it will turn tough collagen into tasty gelatin much faster.

Regulating the pressure/temperature of a PC is as simple as bringing it up to full pressure on high, and then turning the gas/electric hob down low; it requires little energy to maintain as there is no loss of steam. Even if you have to adjust the temperature now and again to keep the pressure right, you're braising for a much, much shorter amount of time.

Oh yeah - and you can buy a good large PC for the cost of a 2 quart Le Creuset Tagine (about $200?) 

More information on the science of pressure cookers (and a great video) can be found here:
http://www.chefsteps.com/activities/pressure-cookers

Recipes, including adjusting conventional recipes for use in a PC can be found at hip pressure cooking dot com.