I think I figured out a way to uv unwrap meshes for the mesh data resource editor. Started implementing it. Mesh partitioning is done.

game/addons/mesh_data_resource_editor/plugin.gd

@@ -113,9 +113,146 @@ func set_axis_z(on : bool) -> void:
 		g.set_axis_z(on)
 
 func uv_unwrap() -> void:
+	var mdr : MeshDataResource = null
+	
 	if current_mesh_data_instance && current_mesh_data_instance.mesh_data:
-		current_mesh_data_instance.mesh_data.uv_unwrap()
+		#current_mesh_data_instance.mesh_data.uv_unwrap()
-#
+		mdr = current_mesh_data_instance.mesh_data
+		
+	if !mdr:
+		return
+		
+	var mesh : Array = mdr.get_array()
+	
+	# partition the meshes along the already existing seams (no new vertices)
+	var partitioned_meshes : Array = partition_mesh(mesh)
+	
+	for m in partitioned_meshes:
+		m.print()
+		#m.unwrap()
+		
+	
+	
+	
+	
+class STriangle:
+	var i1 : int = 0
+	var i2 : int = 0
+	var i3 : int = 0
+	var index : int = 0
+	
+	func set_indices(pi1 : int, pi2 : int, pi3 : int, pindex : int):
+		i1 = pi1
+		i2 = pi2
+		i3 = pi3
+		index = pindex
+		
+	func is_neighbour(pi1 : int, pi2 : int, pi3 : int) -> bool:
+		var c : int = 0
+		
+		if i1 == pi1 || i1 == pi2 || i1 == pi3:
+			c += 1
+			
+		if i2 == pi1 || i2 == pi2 || i2 == pi3:
+			c += 1
+			
+		if i3 == pi1 || i3 == pi2 || i3 == pi3:
+			c += 1
+			
+		if c >= 1:
+			return true
+		else:
+			return false
+		
+	func print():
+		print("[ Tri: " + str(i1) + ", " + str(i2) + ", " + str(i3) + ",  " + str(index) + ", ]")
+	
+class SMesh:
+	var indices : PoolIntArray
+	var vertices : PoolVector3Array
+	var uvs : PoolVector2Array
+	var triangles : Array
+	
+	func is_triangle_neighbour(tri : STriangle) -> bool:
+		for t in triangles:
+			if t.is_neighbour(tri.i1, tri.i2, tri.i3):
+				return true
+		
+		return false
+		
+	func try_to_merge(o : SMesh) -> bool:
+		for t in o.triangles:
+			if is_triangle_neighbour(t):
+				triangles.append_array(o.triangles)
+				return true
+				
+		return false
+		
+	func unwrap():
+		pass
+		
+	func print():
+		print("[ SMesh:")
+		
+		for t in triangles:
+			t.print()
+		
+		print("]")
+		
+	
+func partition_mesh(mesh : Array) -> Array:
+	var meshes : Array = Array()
+	
+	if mesh.size() != ArrayMesh.ARRAY_MAX:
+		return meshes
+	
+	var vertices : PoolVector3Array = mesh[ArrayMesh.ARRAY_VERTEX]
+	var indices : PoolIntArray = mesh[ArrayMesh.ARRAY_INDEX]
+	
+	if vertices.size() == 0:
+		return meshes
+		
+	if indices.size() == 0:
+		return meshes
+
+	var tricount : int = indices.size() / 3
+	
+	for it in range(tricount):
+		var iit : int = it * 3
+		
+		var tri : STriangle = STriangle.new()
+		tri.set_indices(indices[iit], indices[iit + 1], indices[iit + 2], it)
+		
+		var found : bool = false
+		for m in meshes:
+			if m.is_triangle_neighbour(tri):
+				m.triangles.append(tri)
+				found = true
+				break
+				
+		if !found:
+			var sm : SMesh = SMesh.new()
+			sm.triangles.append(tri)
+			
+			meshes.append(sm)
+
+	var changed : bool = true
+	while changed:
+		changed = false
+		
+		for i in range(meshes.size() - 1):
+			if meshes[i].try_to_merge(meshes[i + 1]):
+				changed = true
+				meshes.remove(i + 1)
+				break
+
+	for m in meshes:
+		m.vertices = vertices
+		m.indices = indices
+		m.uvs.resize(vertices.size())
+
+	return meshes
+
 #func forward_spatial_gui_input(camera, event):
 #	for g in active_gizmos:
 #		if g.forward_spatial_gui_input(0, camera, event):

game/project.godot

@@ -286,7 +286,7 @@ window/size/ui_scale_touch=1.0
 
 [editor_plugins]
 
-enabled=PoolStringArray( "res://addons/Godoxel/plugin.cfg", "res://addons/bone_editor/plugin.cfg" )
+enabled=PoolStringArray( "res://addons/Godoxel/plugin.cfg", "res://addons/mesh_data_resource_editor/plugin.cfg" )
 
 [ess]