That's right. With largesse you need to have such mechanics. It is, after all, all about making a fuss about something. It could be as simple as choosing between "trading" and "gifting", where gifting always generates a message, while trading never does, even with 0 "price".
The same could work with handing out gold for whatever reason. If you donate to the temple, you could do it discreetly or with trumpets and trinkets. If you choose the trumpet, it does not count towards your balance in a guild or religion. The game should not follow exact numerical amounts with largesse when choosing whether to give an increase in a stat or not, but would rather make a comparison with the weekly income average for several previous weeks. It's not the total gold at hand which counts, but the income.
It gets a bit trickier with gold transfers between nobles. Again, you could either do it discreetly or with a great pomp. Or to do it the other way around, an individual could declare a need for a certain amount of gold with reasons as to towards which end the gold is needed. Then whoever in the vicinity of a bank could make that transfer and it would be made public. Again, if a knight needs 100 gold for refit, it is peanuts for a duke, but if another knight chooses to hand it out, it would give him a much greater stats bonus.
Sure, this could be gamed such that a wealthy duke gives 1000 gold to a knight who in turn proceeds to hand it out lavishly. But such behavior would be eyed with suspicion by the players, and a corresponding hit to honor would follow. Besides, if largesse was tracked as a value relative to the largesse of the other realm members, you could never increase largesse benefits for all the members of the realm.
Actually all of the three stats could be seen as PvP within a realm, where someone making a gain causes someone else to gain a loss. Largesse would be one way for a new knight to begin to make a name for himself, which would alert the other nobles to outbid him with more largesse. "No no no, I insist, I want to pay for the enlargement of the guild hall! - Very well, you do that, but I will enlarge it even further!!" Which in real life is ridiculous, but for us in BM it could be fun.
There is still the question as to what high largesse would gain for a character in addition to his reputation. One possibility could be that high largesse would have a positive influence on the "honor marks" I have mentioned earlier. You could not influence your own honor directly, but you could influence how much honor you can give to the others.
Or you could see the stats as a threefold split where each stat represents how different groups see you. Honor tells you your reputation among the nobility. Prowess is how the soldiers see you, and largesse is how the peasants value you. With high largesse then you would get morale bonus for the peasants under your control. Ie. if your estate covers 30%, your largesse would influence the 30% of the peasant morale in the region. If you are a lord, you influence the whole region. If you are a duke, your largesse influences the entire duchy. And if you are a ruler, it is the whole realm.
Additionally it could be that knights with high largesse would be somehow better able to deal with the peasants. Their police work would be more effective, as the peasants would help them. Their civil work would be more effective. They would get quite detailed information from the peasants, and perhaps some useful rumors as well. Perhaps they would occasionally even gain volunteers from the peasants to join their units.
Granted this is not entirely logical, as the actions that gain you largesse might not necessarily have any benefit towards the peasants. But it could work the other way around in such a way that looting, harsh courts and such could get you a negative largesse hit. Perhaps characters with low largesse would be more effective in such harsh actions. Again, not entirely related here, but largesse could fill that role.
Perhaps, indeed, having average largesse would also give some benefits. Or rather, having largesse in the extreme in either way would give some very specific benefits, but more penalties the other way around. You could either be loved by the common folk or have them in your iron grip, but not both.