ok I seem to have found the issue...
It seems that placing the rsc in the \WorkSpaces\xxxx\Standards\Symb in ORD adds the linestyles to the linestyle list but results in them not being displayed correctly (eg. solid line in 2d, invisible in 3d). If I put the rsc file in \Organization\Symb then it works perfectly.
The workspaces symb location has been defined in the cfg (MS_SYMBRSRC) and seeing as it is being added to the linestyle list and previews correctly in the editor I don't understand why this isn't working. I have tried putting the rsc in the workspaces location for Microstation Connect and it works fine.