Create an Autodesk Oracle based feature source and depending on the type of data just save the FeatureSource over top of the Topobase one. I'm not sure if you have to rebuild the layer or not depending on what styling definition it was using. This might allow you to see the layer. The symbol library can be used by other resources, even if you can't preview the layer you published. You could just create a new layer and assign it the symbol based on the new feature source.
If all you really wanted was to get the symbols you could use Topobase to create a new sdf with a single point or set of points and assign the blocks to them. Publishing that will get you your libraries. In that case you don't care about the map and layers being published.
An alternative I have not tried is to copy the provider files from Topobase into the MapGuide Studio and Server install and modify the providers.xml file to add the new provider. I'm not sure which files are required for this.
Alternatively if you have Topobase and Topobase web running you could publish to Topobase web and export a package of the folder containing the symbol libraries. You can then import the package to MGE. Again layers and feature sources would probably not work, but you could get just the symbol library.
Dave