@lobaluna I’ve never bothered with xinitrc or xprofile. In my mind, this might be appropriate if you were starting from scratch (think Arch) but for LXDE and LXQt (and just about every other mature desktop environment), with the Freedesktop Autostart Specification being implemented, it seems a little low level.
In both desktop environments, their respective session manager allows for the autostart of an arbitrary command, which in this case would be setxkbmap -option compose:key. Options for this command can be found in the xkeyboard-config manpage, the relevant items here being:
Position of Compose key
┌──────────────────────────────────────────────────────┐
│Option Description │
├──────────────────────────────────────────────────────┤
│compose:ralt Right Alt │
│compose:lwin Left Win │
│compose:lwin-altgr 3rd level of Left Win │
│compose:rwin Right Win │
│compose:rwin-altgr 3rd level of Right Win │
│compose:menu Menu │
│compose:menu-altgr 3rd level of Menu │
│compose:lctrl Left Ctrl │
│compose:lctrl-altgr 3rd level of Left Ctrl │
│compose:rctrl Right Ctrl │
│compose:rctrl-altgr 3rd level of Right Ctrl │
│compose:caps Caps Lock │
│compose:caps-altgr 3rd level of Caps Lock │
│compose:102 <Less/Greater> │
│compose:102-altgr 3rd level of <Less/Greater> │
│compose:paus Pause │
│compose:prsc PrtSc │
│compose:sclk Scroll Lock │
│ │
└──────────────────────────────────────────────────────┘
It certainly seems that it’s an omission not to allow for keyboard options, as there are quite a plethora. There’s an upstream issue that smells like a request to select at least one set of options.