Here's some notes that I've gleaned from dealing with three different intelligent mulitport cards. There are a number of simpler, stupid ones (that are basically a bunch of 16550 uarts glued onto the pci bus), but those I'll not go into here.
Typically, there massively expandable cards have the UARTs builtinto the expansion cards. There's a bridge that acts as a more traditional bus bridge and the uarts typically are on some kind of bus behind that. Typically, there's no information about which uarts you need to look at. The Cyclades Y card driver has to loop through all the UARTs that it has found (but things aren't so bad since each uart controls several ports). The stallion cards vary a lot, but typically they are the same way. Some of the cards can eliminate the loop because they only have one 'pannel'. The pannel handlers get the state changes for the ports and harvest/send characters.
Some uarts are smart enough to some of the character baking that's done in non-raw mode. Only a few drivers try to take any advantage of this, either here or on other free source OSes. While terminal rooms are a thing of the past, some of the uarts can do much ppp processing which might be useful. In any event, chances are good that it wouldn' tbe worth the effort.