hmmm...
I believe the phone software is around 50MB, which must be transferred during the update (so on the slow computer).
The pc software ("NSU") is also around 20-30MB, but can be downloaded with another computer and then transfered e.g. with usb stick
But downloading 50MB over 64kbps line, will last between 1,5 and 3 hours (assuming e.g. a transfer rate of 5KB/s, you get 10000 seconds = 2hours 46 minutes)
Theoretically possible, unless NSU is cutting the line or anything during slow downloads. Also, if the transfer is interrupted, it will not be continued from the cut-off point, but rather restarted.
Bottom line. I would not recommend this.
My Nokia history: 5310 (Sep 2008), 5300 (Jan 2007), 6610 (June 2003), 3310 (2001)