A 1500-lb aircraft pulling six g's puts a 9,000 lb
load on the spar whether the wingspan is 25 feet or 50 ft. Unless
I've forgotten something fundamental, you'd have 2250 lbs of tension

Either I'm mis-reading or you have forgotten something fundanental. The length
of the wing does have a very real effect on the spar attachment points.

A quick example. Pick up an 8 lb sledge hammer by the head at arms length. 8
lbs in your hand. An easy thing to do. Grab the handle half way and hold the
head out away from you again. Bit of a strain on the forarm muscles isn't it.
If you go to the gym on a regular basis try again at 3/4 of the handle.

Another visual aid is a gymnast on the rings trying to do an "iron cross".
Almost anybody can hold themselves up on the rings with your arms locked at
your side but as your "wing span" increases so does the strain on the upper
body muscles.

The same thing happens to spar attach fittings. My little 600 lb 16 ft span
canard has over 100,000 lbs on its little spar caps at the fuselage at 6 G's.
3 is about all it will ever see unless I bounce it real bad.


I hadn't considered laid-up carbon fittings.

Forget hand laid carbon. The end result is generaly no better than E glass.
Graphite takes considerable process control to take advantage of it's full
potential.