I've been playing with this and think I may have stumbled upon the solution. Other posts have mentioned mc-tool, which is not by default installed on the N900.

However, it is available in the package "libmissioncontrol-utils" and can be invoked with

mc-tool request <account name> <presence>

It also has a list command showing the telepathy accounts on the system, so to set everything bar the ring/tel account offline (used for voice calls), we can just

mc-tool list | grep -v "ring/tel" | awk {'print "mc-tool request "$1" offline"'} | sh

Replace "offline" with "online" to reverse the process, et voila.

Usual disclaimers, works for me, use at your own risk, and check your commands before you pipe them to sh!

It should be noted that while the status bar icon behaves as expected, this does not seem to update the global presence setting found within the status menu, so if you go offline, each account will show "network disconnected", but the online button will remain highlighted. You can just hit "Save" and it will put you back online as expected.