Just to confirm - you're trying to force a user to open a link in IE, independent of the browser they are using to view the mashup (in your example, Chrome)?
I don't think that this is possible - given that the browser choice is set at the OS. Also, what if your users don't have IE installed? Or they are on a Mac, *nix, etc?
Perhaps if you can give more reason as to why you would like to do that, I could help advise a better solution?
I have the same issue. and my reason is Active-X. I need to show some reports in SSRS that uses the SSRS Active-X control to print the report. However, Google Chrome does not support Active-X and the WebFrame Widget is generic. If I could have a WebFrame Widget that emulates specifically IE, it should work.