Well much to my surprise, I was able to scout all the way through the Dwarf Lands, Underground Sea, and Demonis. I haven't uncovered all of the map or scouted out all recruitable troops (and available items), but I have a decent idea what's going to be available when I reach these areas. The only tricky part was getting past the guards to Demonis, but even that wasn't very hard, though it seems clear that the level is designed to prevent you entry to Demonis. Also, I haven't tried to get back.
I'm doing this strictly for scouting and have gone back to my game in early Western Islands, lol. I will still consider trying to take out Lucky James reasonably early, then going to Darion/Creiston/Hadar to pick up Lina and maybe some Griffons. I'll have to decide what kind of losses I'm willing to take against Lucky James since I'm really trying to minimize losses. Not a no losses campaign, but have only had single-digit losses since the Tutorial. We'll see.