I was able to get the driver using your method (although I had to clobber the Upgrade Notifier to do it; see details here: Upgrade Notifier interferes with acquisition of additional drivers ).
Unfortunately I seem not to be in the clear quite yet. The computer now connects to wifi but drops it in less than two minutes and always drops it before I can get a webpage with Firefox.
There seems to be some history on Broadcom drivers; e.g. here: How to install wifi driver? - #6 by That_Random_Guy