Unfortunately the Ruby API doesn't implement any API to select multiple files in a file dialog.
Further more, if you use the Importer
class you are stuck with the file dialog it displays. It's single select only.
If you know all files in the folder should be read you should let the user pick a single file, then extract the path from that and read all the files from that directory.
If you drop the Importer
class you can craft your own alternative. The best would then to create a Ruby C Extension that calls the OS API to display a multi-select dialog.
Alternatively, you can create a WebDialog that display the files and let the user pick multiple files. But that means you'll have to create all the UI from scratch and it'll not lok like the native OS file dialogs.
http://www.sketchup.com/intl/en/developer/docs/ourdoc/webdialog.php
https://github.com/thomthom/sketchup-webdialogs-the-lost-manual/wiki
If you are making a Windows only plugin you can make use of the drag and drop feature of HTML5. Though this also has drawbacks of requiring the user to have a recent IE version. (OSX hides the WebDialogs when SU isn't active - so you cannot use it as a drop target when you drag files from Finder.)
I made a proof of concept a while back: https://github.com/thomthom/DropZone