Not Barons of Hell, just ordinary barons. If you are a name-level landowner, how do you earn experience points?
On the previous post, Neophage suggested that a variant of bragging about false conquests could be a useful source of XP for barons. The way I'm interpreting it, that would mean spreading word about your deeds to areas that haven't heard of them before, so that you effectively get double or triple points for monsters already slain.
Assume that PCs get the original XP from word spreading through your home base (1 hex max.) They can then go to a new hex and personally brag about their deeds, or hire a bard or town crier to spread the word; on a 5+, it's a successful campaign and the PC earns half XP.
The downside is that people far and wide start expecting you to live up to your old accomplishments. Beowulf has to go and fight that dragon, because everyone's heard the story of when he killed Grendel and Grendel's mother.
Another source of XP for barons would be related to the idea of treating areas explored or reclaimed as treasure. Income over and above the base tax rate, brought into a barony by investments, becomes XP. Also, income from tribute or pillaging.