I spent a bunch of time looking into the options when trying to find a solution to the stupid cellpipe reboot issue.
It should be upgradeable via TR-069. You would need to setup a TR-069 server and get the device to access it. The trick there would be that the device will most likely only access the server over the dsl link so you would need a dsl "server" (not sure what the correct term is and too lazy to look it up right now). Some DSL modems can be used point to point (ie one modem connected to another modem). I'm not sure about the sagemcoms.
I believe that there is a free TR-069 server implementation available (it is basically just a web server). I also believe that you should be able to find the address that Bell is using for this so that you can fake it. I think it was actually mentioned recently in the cellpipe reboot thread. Otherwise check the logs on the modem to see if you can spot it there.
Does the sagemcom have a console port like the cellpipe does? It may have one but with no external connector (check the board for a header).
There was also a guy in the telus forum who updated the firmware on the telus version of the cellpipe (not quite the same as the bell one) who seemed to have some experience with this sort of thing.
If you can get any type of console access i.e. telnet, ssh or serial then you might be able to get enough access to upload from a TFTP server. I work with enterprise VoIP equipment where this is common but I'm not sure about DSL modems.
They seem to use this method to update their modems. marknotmarc on the forums here reported his cellpipe being remotely updated so I would expect that the same applies to the sagemcoms. It seems to be the standard way of managing them nowadays. The catch of course is that you need to be on the DSL side to do it which is difficult for the average consumer...