Beyond portals and (long) sea voyages, how else could I get characters there if I needed to?
Lots of ways!
Time in L4W is pretty flexible in the hands of DMs. There's no problem at all with having a replacement PC set off on a long sea voyage for some other purpose (e.g. hired as an escort to a throwaway NPC merchant), get lost or thrown overboard in a storm, and wind up on the shore of a random island, which just happens to be the one the rest of the party are on. If it takes a month of in-game time, who cares? It can still be done in a handful of posts.
Daunton is also known to be close to the Feywild in places, which doesn't quite work the same way as the normal world. Maybe what's an island here is just another grove of trees there. Or simply not as far away.
There's the Maze and the Depthworld, which are awesome concepts that have been under-utiilized so far, both of which are connected to the surface world in ways that are not well understood.
You could have the PC kidnapped by the Big Bad and kept prisoner in his dungeon, ready to be rescued by the rest of the PCs. Again, time is flexible; they can be kidnapped and held for months in game time, and still rescued by the PCs a day after you recruit them in real time.
You could simply not explain how the PC got there. Describe a hazy, drunken evening, a bevy of attractive, giggling ladies (or gentlemen, if that's your thing), and darkness. When the PC wakes up, they have a splitting headache, have no trousers on, and are on the island you want them on.
You can find a reason why the PC would already be there on the island, e.g. visiting relatives or old friends, or, if their background is appropriate, they could simply have grown up there and not have visited Daunton yet.
I think, in general, my favorite way to go about this is to invent a minor side quest for the new PC, unrelated to the main quest, such as, "Find out what happened to my brother/cousin/old school buddy", that gives the PC a reason to travel where you need them. Let them travel there by whatever means they see fit, taking however long it takes, and declare that they just happen to get there when you need them. Then work the side quest into the main quest somehow. Maybe the long lost cousin is now married to the NPC who hired the rest of the PCs. Or is a corpse in the lair of the Big Bad. Whatever.
JNC did work himself into a bit of a corner by having an island that is hard to approach, and where nobody lives. If I had to get someone there, I'd probably go with shipwreck survivor, or maybe kidnap victim. Or, heck, both.
Since we don't worry about time continuity that much, even a long sea voyage may not be a huge issue - they can be in the main tavern but arrive at the faraway island the same real-time day, so what. 
