Those are both very valid concerns, and I'm not sure my suggested approach could be easily adapted to address either of them.
No, it is not possible with uber-uploader. There has been some discussion on the sourceforge forum for the project about the addition of such a feature, but the author has not undertaken to add that ability to the program (and has indicated he feels it would be very difficult to do so).
Additionally, a "quick search" through the more popular script archives produced only one upload script that claims it can do this (an ASP application); there may well be one, or more, out there - I just have not located it.
I have not searched for a "drag and drop" interface implementation, so I can't comment further on that, though I suspect something with a "nicer" user interface might be "out there some where".
In my situations, this function is usually used to send a very few files at a time, and I have found that the uber-uploader interface, which allows"several" files to be included by multiple "browses", has been acceptable for my users, but I understand that your needs may be different.
That is stated a little bit differently than your earlier posts, and the difference could be significant.
If you are doing all the uploading, and only require that the system be set up so the your "suppliers and customers can download them again", then just setting aside a directory under your own user in a web accessible location should address that need (you can connect via an FTP client with whatever user interface you like and enjoy the resume transfer function).