Wagner, who said the plans were still at the planning stage, told The Johannesburg Sunday Times: "It's only a proposition on our desk.
"There were a host of issues that need to be taken into consideration before a decision is made."
Stewart has warmed to the suggestion, but confirmed the South African and New Zealand Rugby Unions, and the Sanzar Tri-Nations governing body, need to sign off the deal before approaching English authorities.
He told The Christchurch Press: "Conceptually, an away game played in that part of the world (the UK), as part of the 2008 round robin, is very appealing and has been for some time.
"But there's a lot of work to be done, certainly by the Stormers in the first instance, because, effectively, it's their game.
"I wouldn't say it's likely at this stage because of the amount of work that needs to be done, but it's in line with where we see the opportunities for the Crusaders."
The six-time champions play pre-season games in Melbourne and Sydney this month.