I'm curious if there is any lore about what sorts of activities specifically that a shinobi might engage in that would be illegal if being a shinobi wasn't already illegal.

Specifically, I'm playing in a game right now as a Soshi Illusionist; the rest of the players have uncovered my identity, and I assume if one of them doesn't kill me that there will be questions as to what a Scorpion is doing dressed as a Unicorn in the middle of Crane-Lion negotiations over Toshi Ranbo.

So far as I know, the things they can definitely nail me on aren't actually illegal, just not terribly honorable: I'm at Status 60 and crafted a persona at about half that, and impersonating someone of lower status as far as I know isn't a crime; I imagine the Unicorn won't be too happy about it and might have duels challenged, but the act itself isn't a crime, correct?

tl;dr: I'm on the hook for disguising myself as someone of lower station and a different clan; is that against Imperial law?

Probably. Probably not imperial law per se, but the important flaw in your 'it's-dishonourable-but-not-illegal' argument is that your presence in and travel to Toshi Ranbo was authorised by someone.

If your travel papers refer to you by your false identity, then you are travelling with no papers and no permission from the local lords, which would definitely be accepted by most factions as illegal.

It depends! 😀

Travel papers are a technicality they could nail you on. But if you are status 60 I would think you could get someone (an Emerald Magistrate?) to issue papers to you under your real identity and your false identity. How discrete are/were you?

Being a spy for a clan is not the same as being a shinobi. Its not honorable, but no one is going to have an existential crisis thinking about the celestial order and of how Rokugan is supposed to work because a Scorpion got caught sneaking around. It might be different if you got caught "in black pajamas" sneaking into somewhere you weren't supposed to (and...OMG someone died by poison!).

Your honor definitely takes some kind of a hit by spying in the first place and your glory takes a hit for being caught.

Is it illegal? Not exactly ... I'm sure some buerrecrats try to nail you on something ! But that's because a lot of people have become ticked off at you. And now want a pound of your flesh. That's not necessarily because of what it says on a moldy imperial scroll.

I personally would claim to have been seeking a romantic liaison with someone, and then swear you'd rather fight a hundred duels than share her name. At least that sounds like something a little north of disgraceful.

Good luck! I hope it worked out!