Well sucess!
Hardly any problems (well as far as I can see anyway).
The aluminium version of the W126 (actually it's a 107 part according to the part number) trailing arm most definitely fits in a W123 sub frame. Here's a picture to prove it.
I've only done a rough check by measuring between the back edges of the rims (I used the same W123 disc brake and steel wheel that was fitted to the W123 parts) but there seems to be no difference between the two trailing arms in the way they fit into the sub frame. Here's a picture of where I was making my rough measurements for comparison =>
All is not 100% straight forward though.
Problem #1
W123 brake caliper does not fit on 107 / 126 aluminium trailing arms
Problem #2
Hand brake cable looks slightly different on the aluminium trailing arm. W126 cable is shorter than W123 sedan equivalent (even though they are obviously of a very similar system).
But these are pretty trivial problems - they could very easily be remedied.
Please note - the car is still in bits at the time of writing this post. I have not fitted the sub frame to the car and put the springs and shocks in place just yet - there could be another hitch...
...you have been warned (if you fancy doing this yourself).