I wonder if it's possible to cover yourself in strong enough mundane magic-dampening-or-repellent material to become invisible to Magesight. It would not be applicable to Wizards due to it interfering with their actual magic, but I could see people (in this example, Clan Eshin) outfitting their mundane spies or scouts in such.
Wait a minute, Magesight see winds, winds are disturbe be emotions, and the Eshin learned from Cathay.
Could the trick be not to actively hide from Magesight, but reach a state of Zen where you meld with the background ?
In the RPG (at least in 2e), the Lore of Stealth does have a spell,
Pelt of the Assassin, that lets the user have very good mundane camouflage as long as they stay still. It wouldn't be a
huge stretch for someone to come up with something like what you're saying based on that.
But it would probably mean Eshin-friend wasn't a Sorcerer: We
know you need to cultivate a particular mentality for interacting with any Wind. For Ulgu that'd be a mentality prone to deception, trickery, plot, etc, and for Eshin Sorcerers you'd add the megalomania, self-importance and paranoia of Dhar.
I'm going to headcanon this as the reason Mathilde at quest start had such an anemic spell list, when she clearly has a good amount of talent.
And probably the foundation why she's really, really careful about BM spells (though the constant explosive miscasts she's seen are what built on that foundation).
I will also headcanon this. It's not a stretch to imagine this playing a part in things.