Is there a lathe that can put threads on the end of a barrel while holding the barrel (or upper) stationary, thus eliminating disassembly of the sight base, handguards, barrel nut and etc.?  It seems like someone would have invented this by now.    

You need to remove the barrel from the upper, but not the FSB. Here is the thread of mine being done a few days ago.  
www.ar15.com/forums/topic.html?b=8&f=24&t=185766

MAYBE, you could set up a whole upper in a chuck and and steady rest/live center configuration and thread the barrel but a few things get in the way of that:

1.  Potential damage to components due to the clamping force needed to hold the unit tight to ensure proper threading.

2.  It is WAY too easy to disassemble an upper to justify trying something like this.

3.  The time, effort and money to do such a limited task with a lathe makes it not cost effective.

Quoted: You need to remove the barrel from the upper, but not the FSB. Here is the thread of mine being done a few days ago.   www.ar15.com/forums/topic.html?b=8&f=24&t=185766

Good link, but I have a question:  Did the live center do any damage to the muzzle/crown?  I was planning on doing it this way, but was concerned that the live center would ding the muzzle/crown and affect accuracy.

Nope.  The live center on Mikes lathe is very smooth, it made no visible marks on the crown.  Granted I was not doing it on a match gun. One of the barrels I has threaded was my 20" HBAR that I am a little concerned with accuracy however I am not shooting 1/2 MOA with iron sights anyway.  The other was an M4gery which I rarely shoot over 75 yards.  If it were a $400 Kreiger barrel or something of the like that I have been a bit more concerned.  Worst thing is you would have to run a crown tool over it.  

Thanks for the replies guys, esp. SNIPERm88 for the nice link with great pics.  Guess I'll try to find a strap wrench and spanner to breakdown my floating handguard and barrel nut/lock ring.  I don't mind leaving the lathe work to the gunsmith but I like to do my own breakdowns and assemblies so I can take only the critical part to the shop, in this case, the barrel; but I keep accumulating special tools for specific guns that only get used every few years.  Guess that's the price of satisfaction.  :)