You could probably use a different manufacturer's rack bag if it fit.

But I'd also recommend looking into getting the mounting block that screws into the Brommie's headtube, and getting the lightweight S-frame from Brompton that mounts on said mounting block.

From there, you can cobble together any kind of bag onto the S-frame that'll work. I fitted my $4 rummage sale bag in just that fashion in post #2 of this thread: http://www.bikeforums.net/folding-bi...l#post16093787

The neat thing about this arrangement is that the bag does not move as you turn the handlebars in either direction, which keeps a nice constant center of gravity. And the quick-release system for the S-frame/mounting block is very easy to use.