I don't think this is possible.
How good are your programming skills? You could try and write a
function to reference a tiddler within a workspace, maybe hijack the
[[]] macro and extend it so that if you put [[tiddler@workspace]] it
would recognize it. But you'd have to add some php scripting to grab
the proper tiddler from that workspace, so it's not just javascripting
you'd have to do.
If you wanted to search all workspaces, then you would need to
completely redo the search, as the current search only searches what's
been loaded (I think, could be wrong)
K.