Fantastic, it worked.

Now I can at least borrow a computer, download the ascm file from the library, use my own portable ADE to authorize and download the actual book and then simply connect my device and tell the portable ADE to transfer it (or just copy the book to the device).
I noticed a nice little thing though. When I started the portable ADE from the usb-stick, the existing MyDE folder on my computer changed temporarily to "My Digital Editions.BackupByADEPortable" (with all books inside) and the created a new temporary My DE folder, in which my downloaded book appeared. When I shut down the portable ADE, the temporary portable MyDE folder disapeared and the temporary My Digital Editions.BackupByADEPortable renamed itself to My Digital Editions again.
Nice little function if the borrowed computer already has ADE and a MyDE folder installed.
Another thing I noticed was that if a book has already been borrowed and authorized by another ADE, even it by the same Adobe ID, I got this message:
"Unable to download
Error getting License Server Communication Problem:
E_LIC_ALREADY_FULFILLED_BY_ANOTHER_USER"
Anyway, it worked with new borrowed books.
This should be a sticky of how t solve this kind of problem for people on the move without phones or with older phones like mine that can't run ADE. While we wait until we get ADE in the devices.... And when they do, would it be possible to have them on the older devices, as long as one update the firmware?