There is no way to directly attach the metadata to the .lyr file, because the file doesn't support metadata inside it. In desktop, there is an associated .xml file that contains the metadata. When you upload a .lyr file to ArcGIS Online, the xml file doesn't come with it. You could input the metadata as part of the description of the item inside ArcGIS Online.
ArcGIS Server map services allow you to include some simple metadata that is taken from the description of a map document. Additionally, if you were to reference a map service as an item in ArcGIS Online, you could add some additional description that is part of the item, just like the lyr file above.
As I understand it, ArcGIS Server 10.1 will support authoring full metadata (e.g., FGDC, ISO) and this will be available as part of map services.
Hope this helps,
Mike