The SC reroll can be used when he is on an AC undertaking a ground attack order as it is one formation at that point.
Can't be used for spaceships as the SC is never part of the Spaceship formation.
Here's the FAQ
Quote:
Q: If a formation is offboard (awaiting
teleport, in a transport aircraft, etc.)
can any special abilities of units in
that formation be used?
A: No. Special abilities of offboard units
may not be used. A specific exception is
made for abilities used to affect the
activation of the formation they are in.
For example, an Eldar formation with a
Farseer is held offboard in reserve and the
Eldar player retains the initiative to
activate this formation. Even though the
formation is offboard the Eldar player can
use the Farsight ability of the Farseer in
that formation to negate the penalty for
retaining the initiative. Similarly, a Space
Marine Supreme Commander may use the
Supreme Commander ability to re-roll the
command check to activate the formation
they are in if it was offboard but could not
be used to apply that same re-roll to a
formation that was onboard or to another
offboard formation.
