Its been a while we want to move away from existing Cactus tests for one of our Legacy application. Have been trying to see what you mentioned is possible with Arquillian. Going through all that arquillain docs and discussions, the Arquillian team never seems to be interested about non-modularized applications (i.e. not so cleanly separated modularized application : legacy). There is not much positive hints from any of the developers too.
On a personal note, i feel disheartened that Arquillian doesnt allow test-already-deployed-application. May be will have to dig deep to make a custom made solution.