Your impression that Raspbmc requires internet connectivity for installation is incorrect/out of date.
See
http://www.raspbmc.com/download/ which includes the following (with my emphasis):
"Just want an image without a fancy installer?
Network Image (recommended) – the default installer image, always gets the latest version of Raspbmc.
Standalone Image – If you just want a standalone image that can be booted without a network connection for initial setup, then you can get the latest image here. This image will resize your SD card partitions to use the full size of your card on bootup. This image should only be used as a last resort, say if you live in a rural area and are planning to download the image at work."