Something like this:
void rotatevectorsbyangle(vector angle)
{
vector oldx=v_forward, oldy='0 0 0'-v_right, oldz=v_up;
angle_x = -angle_x;
makevectors(angle);
vector angx=v_forward, angy='0 0 0'-v_right, angz=v_up;

v_forward_x = angx_x*oldx_x + angx_y*oldy_x + angx_z*oldz_x;
v_forward_y = angx_x*oldx_y + angx_y*oldy_y + angx_z*oldz_y;
v_forward_z = angx_x*oldx_z + angx_y*oldy_z + angx_z*oldz_z;
v_right_x = angy_x*oldx_x + angy_y*oldy_x + angy_z*oldz_x;
v_right_y = angy_x*oldx_y + angy_y*oldy_y + angy_z*oldz_y;
v_right_z = angy_x*oldx_z + angy_y*oldy_z + angy_z*oldz_z;
v_up_x = angz_x*oldx_x + angz_y*oldy_x + angz_z*oldz_x;
v_up_y = angz_x*oldx_y + angz_y*oldy_y + angz_z*oldz_y;
v_up_z = angz_x*oldx_z + angz_y*oldy_z + angz_z*oldz_z;

v_right = '0 0 0'-v_right;
}
IIRC the initial v_forward/v_right/v_up must be the parent's vectors (from makevectors - beware pitch sign). the 'angle' arg is the child's angles relative to its parent ent.
You can then use vectoangles2 to compute your child model's angles, but if its an older engine then it'll ignore the up vector and give you a roll angle of 0 which will be glitchy if your parent is freely pitching/rolling.

actual 6dof stuff will require you to make clientside changes so the player's view angles don't get bounded weirdly etc. 

Many thanks, works like a charm!