Hey all! Thanks to the help of Thunder-3D I've managed to work this out.
Here are the steps to get USM working with G8.1F and NewGens:
1. Load G8.1F
2. Load NewGens
3. Load USM.
4. Select the USM shell in the Scene tab
5. Go to the Surfaces tab and with the USM Shell selected, select both of the following:
new_gens_V8_1840_Anus
new_gens_V8_1840_Genitalia
6. Apply the Iray Uber Base to these two surfaces
7. Right click on the 'Body' area for G8.1F, or the 'Torso' area for G8F, and select 'Copy Selected Surface(s)'
8. Right click on both select both 'new_gens_V8_1840_Anus' and 'new_gens_V8_1840_Genitalia' and select 'Paste to Selected Surface(s)'
9. Still with both 'new_gens_V8_1840_Anus' and 'new_gens_V8_1840_Genitalia' selected, scroll down to the 'UV Set' dropdown and select 'Base Female'
The USM shell should be working normally now.
If you change the material used on USM, repeat steps 8 and 9.
I hope this helps and super big thank you to Thunder-3D for helping me figure it out!
We're all Dirty Minded... so let's make some Filthy Art!