Uniques offered by Elder Madox can be rerolled

it is possible to reroll the Uniques that are offered by Elder Madox in the Kriar Peaks, as long as you haven't taken one yet and there is at least another open quest in the Interlude.

The Uniques offered appear to be Area Level dependened, so when you can create a new Instance, that has an other area level than your current, he will offer different uniques.

You can create a new Instance with a higher Area Level by leveling up and completing a a quest in the Interlude.
Since the Interlude Areas scale dependend on your level and the completed quest, creating a new Instance will create an Instance with a higher Level.

https://www.youtube.com/watch?v=Qbkgc9Mkk9I

(ignore my fumbling in with the shrine in the Sanctuary... XD)

I'd guess that the "abuse" potential is limited, since you can only get an Instance with a higher Area Level, when you levelup AND complete a quest. Since there is a limited number of Quests in the Interlude and leveling up will take longer each time...