Blackace, the problem is not all individual light types are available in the control assignment list.
Smeely,
For those events that do not show up for assignment in the Settings>Controls window......
..I have by manually editing the 'standard.xml' file that stores the key/button/axis assignments. The file is stored in the same location as your fsx.cfg file, in the Controls sub-folder. Back up the file before editing so you don't lose your current assignments should your editing efforts 'go south'.
I have assigned my electrical and light switches to the numpad keys, as I don't use them otherwise in 'normal mode'. As an example, I assigned beacon to num 7 by locating the current assignment for num 7.....
<Entry>
<Key>Num_7</Key>
<Down>ELEV_TRIM_DN</Down>
</Entry>
I replaced the event string, ELEV_TRIM_DN, with the one that toggles the beacon switch, TOGGLE_BEACON_LIGHTS.
<Entry>
<Key>Num_7</Key>
<Down>TOGGLE_BEACON_LIGHTS</Down>
</Entry>
The event strings for those lights not listed in the Controls assignment window...
TOGGLE_BEACON_LIGHTS
TOGGLE_TAXI_LIGHTS
TOGGLE_NAV_LIGHTS
TOGGLE_LOGO_LIGHTS
TOGGLE_CABIN_LIGHTS
There are several <SimControls.Map> sections so make sure your editing entries under the <Name>KEYBOARD_MAIN</Name> section and not the <Name>KEYBOARD_SLEW</Name> section.
If you have the SDK installed you can find other events strings the 'fsxsdk.chm' document, Core Utilities>Variables>Event IDs.
Edit the xml file in a plain text editor like Notepad. Be careful editing as missing or extra characters will cause problems.
Backup your edits often so you can fall back if something goes awry.
Use this information at your own risk...

Dave
Note: The other option I can think of is using the registered (payware) version of FSUIPC. Though I don't have it, I believe (though I'm not sure) that it allows you to map buttons/keys/axis to events not available in the sims Control assignment window.