OK I get the idea.  As far as I know SAMP hasn't been used much in this way, my guess is that use from scripts has mostly been restricted to sending commands to external tools rather than acquiring data from them, though I don't have any real data to back that up.  Topcat does not support an MType along these lines.  Ds9 (for instance) however does; there is an all-purpose ds9 control MType [ds9.get](http://ds9.si.edu/doc/ref/samp.html), and if you send it with the argument `cmd="fits"`, it sends you back a URL giving the FITS file that it's currently looking at.

It would be a bit problematic to define a generic (application-independent) MType of this sort, since the form of the identifier you'd use to specify which table/image/whatever you want is bound to be dependent on the receiving application.  It could just pull the "default" table I suppose, like the ds9 one.

It wouldn't be hard to add something like this to topcat, at least as an application-specific MType.  I'll think about it; certainly if I get the impression that there's much call for it I'd probably do it.