*picky* In general, prefer to keep array, array_len pairs grouped. Also mesh is often used for Mesh data so this could be called ... bmesh_override and placed after objects_len.

Passing in the meshes is a bit ugly, we could even split this into two functions, one that takes objects and another that takes a BMesh array - but the overhead of having too identical functions also isn't great, even so - it would be good to note why this is needed and if the objects are needed when mesh overrides are passed in.

Instead of inline comment, this should use more formal doxy:

Something like this:

/**
 * Pack islands of multiple objects.
 *
 * \param bmesh_override: BMesh array aligned with `objects`.
 * This is needed to UV perform packing on objects that aren't in edit-mode (on creation for e.g.).
 * Optional, when non-null this overrides objects edit-mode data.
 * 
 */