After thinking about it for a while, it seems to be doable. The Nemesis data structure needs to be modified.

I never use this feature in Rake, so I'm probably not gonna do it unless people start to shout :) never over engineering, ever!

Nemesis is in a state I'm pretty satisfied, the implementation generally gets out of my way, and falls back from DSL to general Haskell code quite gracefully.

7 releases were made in 3 days, yeah, I'm crazy. Just can't stand knowing that people will get imperfect libraries.

update: I did it, in the 8th release, damn it.