According to https://wiki.secondlife.com/wiki/LlSetLinkGLTFOverrides, "Passing an empty string ("") as the override value will clear that override." This is true as far as I have tested except for OVERRIDE_GLTF_BASE_ALPHA, where setting to "" seems to simply do nothing.
To Reproduce: Rez a prim, add a PBR material to it, and put the following script into it:
default
{
touch_start(integer total_number)
{
toggle=!toggle;
if(toggle) llSetLinkGLTFOverrides(LINK_THIS,ALL_SIDES,[OVERRIDE_GLTF_BASE_ALPHA_MODE,PRIM_GLTF_ALPHA_MODE_BLEND,OVERRIDE_GLTF_BASE_ALPHA,0]);
else llSetLinkGLTFOverrides(LINK_THIS,ALL_SIDES,[OVERRIDE_GLTF_BASE_ALPHA_MODE,"",OVERRIDE_GLTF_BASE_ALPHA,""]);
}
}
You will see that the alpha will set to 0 on the first click, but will not return to its previous value on the second click. The alpha mode WILL revert as intended if you use a PBR material which is not in blend mode, but the prim remains invisible if the material is not opaque.
The wiki page provides a toggle alpha on/off function, but this simply toggles the alpha value between 0 and 1 rather than setting and then removing an override. Where the base alpha value of a material is not 1, this will fail as it will set the alpha to 1 rather than the correct value.