
Not useful unless you've got some truly wonderful propulsion system for
the mother ship that can't be applied to the probes.  Otherwise it's
better to simply launch the probes independently.  The outer planets
are scattered widely across a two-dimensional solar system, and going
to one is seldom helpful in going to the next one.  Uranus is *not* on
the way to Neptune.  Don't judge interplanetary trajectories in general
by what the Voyagers did:  they exploited a lineup that occurs only
every couple of centuries, and even so Voyager 2 took a rather indirect
route to Neptune.


Solar sails are pretty useless in the outer solar system.  They're also
very slow, unless you assume quite advanced versions.