If I'm not mistaken, the isokern & rumsford are fireplaces that you assemble on site and then do your outer covering ( stucco, stone , etc ). While these are OK to look at their benefit has to be the lower cost. I've never installed one because it just seems that most of our clients want something that doesn't look like an insert.
However, I think that some of the design aspects could be applied to a custom built fireplace, just like we will probably follow the Rumsford flue and throat design on our next one instead of your typical indoor fireplace flue and throat assembly.
I've looked all over the internet on good construction techniques for these outdoor fireplaces and the info is very scarce. I still need to learn more but I can say the ones we have finished have all worked just fine. I just started with a reference guide from the Brick Assoc. of the Carolinas and haven't looked up their site but here it is anyway:
www.gobrick.com.
At least that's what it says on their brochure entitled "Good Practice for the Construction of Fireplaces". I figured if I could get an outdoor fireplace to have similiar specs as an indoor fireplace, I should be pretty safe.