I remember reading somewhere that the reason the MeshID of MeshParts are neither readable or writable is due to some weird restriction set in by the way the engine/system works - which isn’t really intentional for the actual plan of MeshParts.
That is to say, it’s very likely that the property will be able to get read (and possibly also written to!) both in studio and online sometime in the future, once the meshparts are further developed.
http://devforum.roblox.com/t/unable-to-assign-property-meshid-it-is-not-scriptable/26623/2
Although for Zeuxcg’s comment found here http://devforum.roblox.com/t/were-getting-uploadable-meshes/24733/63 it seems like the ID may not be wanted as publicly available as some developers do.
I’d wait a bit, and see what happens to the MeshID property. If it turns out to not be readable from online play, that’s a feature request to make for that time. Writing to it in studio would be nice, but some comments from the ROBLOX staffs involved in the MeshPart project may elaborate on the difficulties of that feature.