I have used the idea that IMOTU, Nobles do not lie.
It is a point of honor that as a noble, you never lie.
Referencing the Bene Gesserit of Dune and the Minbari of B5, you can do a lot by omitting certain parts or misleading someone without actually lying.
IMOTU, that shade of deception is perfectly acceptable provided the noble does not actually speak a falsehood.
It has worked very well for me with the few noble characters that have been in my games, it really adds a special dimension to the noble PC character; they have to be VERY CAREFUL when their character speaks.
It also added a bit more grayness with the Zhodani...
The way I handle that is:
It is accepted and laudible to deceive a rival who knows you are trying to do so in a context where deception is considered acceptable(sports, war, politics) because that shows your skill.
It is not accepted to deceive someone in a context which depends on trust(trade or sophant relationship).
A lie is defined primarily as taking advantage of someones trust. Where someone is not being expected to trust you(again in that specific context; they may trust you on normal occasions) it is different.
Stratagems are laudable. Merely lying in your speech is not for two reasons. For one thing it is childish and inelegant. One should deceive with style. The second is that the act of communicating can be considered a sign that trust is restored. Thus The Man Who Never Was was an honorable deception because it had been done with style. The Frenchman who captured a bridge in the Napoleonic wars by coming under a white flag and telling the Austrians that peace had broken out, was just being uncouth. As you point out, when you can mislead someone without "actually lying", where's the fun in lying. IMTU deceptions are expected to be elegant and to make a good yarn.
Among the variations of this are that it is considered fine to deceive a business competitor(by letting his agents think the really good market you found was on a different planet for instance). It is not fine to attempt to deceive a partner in a contract(by letting him think your product was better then it is). Another is that the use of moles is looked upon with jaundiced eye, because moles, unlike ruses, threaten another faction's in-group loyalty and trust and loyalty is one of the most valued of virtues. If you are going to accept treachery from someone else's brother who knows where that will lead. IMTU they would have thought Fredo Coreleone got exactly what he deserved. Of course moles are still used and quite widely(disapproval of them is by nature unenforceable). But it is theoretically not held in high favor.
And finally it is not honorable to deceive members of your in-group. Nor is it laudable to deceive except to protect the In-groups interests. Protective missions like war or police work justify deception, productive missions like trade don't(I got that point from Jane Jacobs' Systems of Survival). Where there is confusion between the two types of missions the sages can argue. An exception to this is of course sport. Poker players are allowed to deceive each other within the rules because that is the point.
IMTU