Não parece haver um caminho, pelo menos não fácil. Estas são as opções preparadas:
$ cat /usr/share/X11/xkb/rules/evdev.lst | grep lv3
lv3 Key to choose 3rd level
lv3:switch Right Ctrl
lv3:menu_switch Menu
lv3:win_switch Any Win key
lv3:lwin_switch Left Win
lv3:rwin_switch Right Win
lv3:alt_switch Any Alt key
lv3:lalt_switch Left Alt
lv3:ralt_switch Right Alt
lv3:ralt_switch_multikey Right Alt, Shift+Right Alt key is Compose
lv3:ralt_alt Right Alt key never chooses 3rd level
lv3:enter_switch Enter on keypad
lv3:caps_switch Caps Lock
lv3:bksl_switch Backslash
lv3:lsgt_switch <Less/Greater>
lv3:caps_switch_latch Caps Lock chooses 3rd level, acts as onetime lock when pressed together with another 3rd-level-chooser
lv3:bksl_switch_latch Backslash chooses 3rd level, acts as onetime lock when pressed together with another 3rd-level-chooser
lv3:lsgt_switch_latch <Less/Greater> chooses 3rd level, acts as onetime lock when pressed together with another 3rd-level-chooser
Então Ctrl Direita (e alguns outros) pode ser girado para uma tecla de 3º nível, mas não para a Ctrl Esquerda