I have a custom xkb symbols file which looks as follows:

// modify the basic German layout to have polish characters
default partial alphanumeric_keys
xkb_symbols "basic" {

    include "de(basic)"
    name[Group1]="Germany - with polish characters";

    key <AD03>  { [         e,          E,      eogonek,      Eogonek ] };
    key <AD09>  { [         o,          O,       oacute,       Oacute ] };
    key <AC01>  { [         a,          A,      aogonek,      Aogonek ] };
    key <AC02>  { [         s,          S,       sacute,       Sacute ] };
    key <AD06>  { [         z,          Z,    zabovedot,    Zabovedot ] };
    key <AB02>  { [         x,          X,       zacute,       Zacute ] };
    key <AB03>  { [         c,          C,       cacute,       Cacute ] };
    key <AB06>  { [         n,          N,       nacute,       Nacute ] };
};

The name of the file is depl. I copy the file to /usr/share/X11/xkb/symbols and it works with setxkbmap depl.

However, I also tried to add the respective menu entries in the "Text Entry" customization. I have modified the file /usr/share/X11/xkb/rules/evdev.xml and added the following section:

<layout>
  <configItem>
    <name>depl</name>
    <shortDescription>depl</shortDescription>
    <description>German (with Polish characters)</description>
    <languageList>
      <iso639Id>ger</iso639Id>
    </languageList>
  </configItem>
</layout>

I have then reconfigured the xkb data with sudo dpkg-reconfigure xkb-data.

It works in as much as that the new layout appears as a viable option in the Text Entry dialog, it can be added to the list of dialogs and is visible in the application indicator:

However, it does not work, the new symbols are not loaded. No errors are reported in /var/log/Xorg.0.log.