Well, it seems to me that this 'blood oath' is the opposite of logic, if there is any logic to be found:
- The only thing I know for sure (so far) that can be lost in the DR is sanity. Maybe that's what your willpower loss would represent?
- However, I'd expect sanity loss for those who attempt to escape the DR as quickly as possible. People are said to become insane because they are lost in the DR. As in: they can't find the exit, therefore they lose it.
- Those who feel comfortable in Dakkru's world wouldn't be afraid, wouldn't become insane. Yet they could be tainted in another way.
As a result, I find it difficult to affect both kinds of characters in different ways. It's as if a stat would decrease for people running and jumping, and not for people reading, working or resting. Could sanity be run by mechanics? And should it, in a PG game?
I'm not against permanent losses personally, but I don't see how to make them both bearable and logical, yet.
Maybe, just maybe...
Once (if) hunger, food, and food rot are implemented, I could see some way: People staying long in the DR would eventually run out of food, and might have to resort to eat local products... with detrimental effects on the stats. But even that may not make much sense. We have to study the DR inhabitants, such as Londris, and see if they suffer any kind of stat loss, either in the DR or outside of it.
Another thing I just realised, from your description: someone running errands between the DR and Yliakum would suffer many permanent losses. Or the same could apply to someone who is unlucky enough to "die" often. Which leads me to: how about a stat cap instead of a stat loss?