Ok, thanks for all the votes, and even more so, for all the explanations. It was very interesting (and at times surprising: people seem to see demons everywhere, which may be very medieval, but it's definitely not the case here).
Just in case you may be wondering, here's what happening in the background (leaving out a few less important details). Basically, there's a (struggle for a) change of leadership in House Mercere. The new up-and-coming faction is a mercantile one that focuses on trade, loans and the like (see HoH:TL, and C&G). They realized the following:
They just can!
a) Through magic, the difference between creating a given amount of wealth for a single covenant, or for half the Order (i.e. for about 100 covenants), is marginal: just 2 magnitudes.
b) The fundamental problem with mundane wealth is then not really creation, but (re)distribution. You can easily create hundreds of thousands of Mythic Pounds in spices, silver, silk etc. But what then? An individual covenant wants food, weapons, lab equipment, books, a little silver to pay for its specialists (and a way for them to spend it) etc.
c) House Mercere already has a vast distribution network interfacing with mundanes. It can ship bulk goods throughout Europe thanks to its portals, and has several Redcaps working as merchants, ship captains etc. So, for them, "redistributing" enough wealth to support the whole Order is not that hard.
d) Note that c) covers the second part of the trade too: acquiring those goods that covenants need, and that are more or less the same for all covenants: food, lab equipment, weapons etc.
So the mercantile faction sees a huge opportunity to gain power for House Mercere within the Order, and by reflection to gain power for itself within House Mercere.
Here's the plan!
House Mercere plans, subtly, to start replacing the Sources of Income of individual covenants with their own largesse. They won't press the issue. But Sources of Income constantly come and go; whenever a covenant looks for a new one, House Mercere will simply freely offer its own largesse as an alternative. From the point of view of the covenant's magi, it means getting what they need without effort, from a party they already trust with their correspondence, so House Mercere expects most will accept. Over several decades, perhaps a century, they expect to be in control of the greater part of the mundane wealth of the Order.
Of course, not all magi will accept. House Tremere will likely keep its own reserves. A few shady, Tytalan covenants will prefer to retain their untraceable wealth for their nefarious purposes. Some will keep on producing their own wealth because of tradition, because they enjoy it, etc. But the more widely accepted "Mercere support" becomes, the more natural it feels to accept it, and the shadier it looks to refuse it -- what do you want to hide, and wouldn't it be way safer to let the Redcaps handle what they clearly can handle?
Once in control of most of the Order's mundane wealth, House Mercere will have enormous power even if it never explicitly pressures anyone. From its purchases, it will have extra intelligence on what's going on in the Order. Covenants will be mildly dependent on it, and likely to defend it in a crisis -- and to help it with magic and votes on a regular basis even without any pressure. In fact, given that they delegate that part of mundane interaction to House Mercere, most covenants will likely slowly come to delegate even more -- asking for House Mercere's information on the mundane world, mediation with nobles and the church, etc. etc. Eventually, interaction with mundanes will really mean interaction with House Mercere for the rest of the Order ... and mundane-powers-that-be will come to believe that the Order is House Mercere.