I was thinking more in the terms of a WP-Members plugin core change.
Most themes will be responsive in the near future due to the onslaught of smartphones and the younger generation choosing these devices vs a full desktop.
Suggest you work with the Responsive theme author Emil Uzelac to make the WP-Members plugin responsive.
Here is what Emil suggested when he assisted another plugin author with getting a photo management system converted to a responsive design:
"There's no standard documentation to "Responsive Design" however it should be pretty straight forward change. All* fixed DIVs should be "converted" to fluid DIVs and that will turn fixed "layout" to fluid one.
and also feel free to peak at my http://wordpress.org/extend/themes/responsive if needed.