OK, I've added support in topcat for a new MType `table.get.stil` which will get you the URL of the current view of a table loaded into topcat.  This is not format-specific, you can choose the format you want (from those offered by STIL/topcat) with the `format` parameter.  The `id` parameter, if present, gives the index number (as shown in the control window table list) for the table you want.  If there's no ID, you get the currently selected table.  It's still not clear to me whether this will get much use, but it wasn't hard to add.  Thanks @taldcroft for the suggestion.  The commit is https://github.com/Starlink/starjava/commit/215fbcd8b2cb502ef39bb5b514d55184a2324339.

However, I'm regarding this as experimental.  It may be withdrawn or modified (especially if other SAMP applications decide to do something similar) in the future.  So I would not encourage relying on it in the astropy.vo.samp interface for now.  But I'll be interested to hear if people do anything useful with it.

You can find a pre-release version with this functionality at ftp://andromeda.star.bris.ac.uk/pub/star/topcat/pre/topcat-full.jar.  Probably, it will be in the next public release v4.1 (should be soon).