I think you can do that with
MeshDataTool, a helper class that you build from a mesh to access its attributes (vertices, UVs etc) http://docs.godotengine.org/en/stable/classes/class_meshdatatool.html?highlight=MeshDataTool
I never used this and its not well documented, but from what I see, it doesn't look complicated to use.
This is a guess on how to use it:
var tool = MeshDataTool.new()
# Initialize the tool from the mesh and a surface index.
# Meshes contain surfaces, which in turn contain vertices.
# I guess you want the first surface, but keep that in mind
# in case you have multi-material meshes
# Then iterate all vertices and whatever
for i in range(0, tool.get_vertex_count()):
var position = tool.get_vertex(i)
# Delete the tool once you're done with it, because it only inherits Object
# (or use .clear() if you want to re-use it for another mesh)