Hello... I'm wondering if there is a way to tell the UV Layout SOP to lock the UV position of certain pieces and layout the rest around these locked pieces?

I have a workaround where I convert the UV coordinates to 3D space to get a 3D representation of each UV islands, then I boolean subtract a grid by the 3D locked islands to extract the available space and then use this mesh as a "Pack Into : Island From Second Input" to layout the rest of the pieces in 3D space and finally copy those new positions back to UV space to get the desired results...

That approach works, but something tells me there must exist an easier, more efficient way to do that? Any recommendations?

UV Layout SOP has a group parameter you can use to specify which prims to lay out

and you can then check Avoid Packing Over Non-Gorup Islands to do what you expect