I like the idea of a "dream" you get by resting in specific location, that was mentioned. It allows you to consume IWD1 in bite size chunks in between stuff you do main campaign and also lets you do IWD1 even during IWD2 or after, because that stuff already happened anyways, you just "observing it". Also there is a lot of things you can do to play with this idea. For example dreaming could be explained like: you encounter part of some artifact, that when you sleep in specific place pulls you to relive it's history and see where missing part ended so you can fix it. Or instead of artifact it could be that you need to relive past of your ancestor. Dream also allows you to easily explain other stuff, like why is your party there with you (you just replace past party members with familiar faces), potential power shifts when going in and out, why you don't keep items (could potentially get them after ending IWD1, like @Leeux mentioned, hidden in some area)
ofc dream explanation may seem like cheap one, but if done with care it can make sense and it solves a lot of issues present in other ways of handling IWD1 content.