I will conveniently avoid any dbus talk, because the why is not so interesting as the how and direct you to this path /var/run/wpa_supplicant. You would probably send SCAN_RESULTS on the socket, you could also initiate a SCAN first to include the strength of stations you’re not connected to. If you want deeper access to wireless, you use netlink to communicate with the kernel (see /usr/include/linux/nl80211.h) and poke some NL80211_STA_INFOs… or the other direction (everything is a file) you just parse /proc/net/wireless without any special permissions for the current signal strength.
Oh… and btw dbus has a simple binary protocol underneath all the XML/interface fluff and uses a UNIX socket.