Well in theory, this is what Guitar proposed:

Code:

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vec_set(temp,vector(entity.pan,0,0));     // Store the angle in temp    
vec_inverse(temp);       // Invert the angle    
ang_rotate(entity.pan,temp);  // Rotate the angle

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Ang_add wouldn't help since it's used for adjusting angles relative to the world axis AFAIK

Cheers

Frazzle