Its a good question. At the moment they area treated like any other ship, but I agree they should have some special status. However, I don't want a complete free pass so you can send your planet-killer to the alien home system because it also has a diplomacy module
Open to suggestions here, but I will maybe go with NPRs ignoring the first 10,000 tons of any diplomatic ship in any non-Core system.
Two things seem relevant to me here.
1) How much the NPR is willing to tolerate aliens inside its borders. That is, how high their Xenophobia rating is. A race with a sufficiently high Xenophobia rating would find the presence of your diplomatic ship in their system horrifying enough that just having it there and trying to talk is only going to make it worse.
2) How militarized your diplomatic ship is. If it's counted as a civilian ship and they aren't xenophobic enough to blast it from the sky for being a ship that's not theirs, there is no issue. The more militarized the ship however, the more problems it creates, and Planetary Protection Value is a stat that could be used to measure how militarized the ship is. I'd say that ship's PPV directly impacts your ability to talk peaceably with NPRs, but that more Militant NPRs are more willing to tolerate armed diplomatic ships so long as it doesn't get too ridiculous.
Actually, I have seriously considering having planets with high tectonic ratings explode if hit with enough ordnance, resulting in an asteroid belt
Maybe in a later version.
That's not how that works.
Very cinematic and hollywood appropriate, but that's not how that works. To blast a planet so hard that it turns into a debris field would require dumping so much energy into it you start exceeding its gravitational binding energy, and that is orders of magnitude greater than any energy release that could come from constant high power earthquakes. You
might be able to start blasting chunks off the planet with heavy enough ordnance, but in that case how tectonically active it is is irrelevant.