Was there ever a Jupiter transit or Saturn transit? I've watched the Venus transit 2004 with a small telescope, a relatively rare event, considering most people can only observe two in their life time.

If E.T. were sitting in a sunny place on a moon of the outer planets,
sipping on a liquid nitrogen drink, would there ever be (has there ever been) a transit of an inner gas giant?

I guess if E.T. were in Sputnik Planum, there would be no transits due to Pluto's orbital plane being inclined too much wrt. the Ecliptic.
Wikipedia actually has many pages on this subject. Transits of Jupiter, viewed from Saturn are rare. The calculations are done from the centre of the planet, rather than a moon, but the principle is clear: Transits of Jupiter are rare.

The next, non-grazing transit of Jupiter visible from Saturn is in 7541. From Uranus in 2714, and from Neptune in 2188.

But probably the most photogenic transit will be the transit of Saturn from Uranus in 2669. 

Saturn will also transit from Neptune in 2061, the only transit which we could live to see. Uranus will not transit from Neptune until 38172, and transits from Pluto or other KBO are even more rare, due to the high orbital inclinations.