I know that you can replace the T/GT500 outer seals without splitting the cases. You can't replace the labyrinth seals that way but from what I have been told they do not usually need replacing. You could replace the outer seals and do a leak down test to check them afterward.So, I listened to somebody, who claimed that you can replace the crank seals with the engine in the frame. That seems bit sketchy, as applying any treebond to seal the cases seems pretty darn tricky if the engine is in the frame, and new seals would do F-all if the cases would leak. Now I'm trying to decide between just running the bike for the summer with the old seals or pulling the engine and replacing the seals on the bench.
The KTM forks repair didn't turn out as ugly as I was afraid, but plenty expensive in any case. Might even get the forks back this week.
You cannot safely replace crank seals without stripping the whole motor on any bike. Early yamaha twins had seals with smooth outers and they changed that with the RD250 (air cooled) series. The cranks seals on an RZ have a rib around the outside to stop them moving.
What I'd recommend is doing a leakdown test. There are some reasonably priced leakdown set ups available and plugs for the exhaust or inlet are available from your local hardware store. On a Yamaha twin you have to block off both sides at once because the lab seal allows air to "leak" from one side to the other.
A leakdown test will show you whether there is a significant leak that should be repaired or if it's good to go. Do it while the side covers are off.
Interestingly enough my 85RZ350 (US model) has different carbs with linked throttle and no power jets.