>> [ Also, I don't see the customization group until I load
>>   lisp/leim/quail/indian.el?  But AFAICT, that's not the case for other
>>   custom groups.  ]
>
> There are no defcustoms in leim/quail/ files.  How about moving the
> defcustom to lisp/language/indian.el?

Hmm, moving it to lisp/language/indian.el brings in warnings about
undefined vars and functions, and an error when dumping.

    In toplevel form:
    language/indian.el:147:31: Warning: reference to free variable ‘tamil--vowel-signs’
    language/indian.el:151:32: Warning: reference to free variable ‘indian-tml-base-table’
    language/indian.el:154:41: Warning: reference to free variable ‘indian-tml-base-digits-table’

    In end of data:
    language/indian.el:143:10: Warning: the function ‘tamil--setter’ is not known to be defined.
    rm -f emacs && cp -f temacs emacs
    LC_ALL=C ./temacs -batch  -l loadup --temacs=pdump \
        --bin-dest /usr/local/bin/ --eln-dest /usr/local/lib/emacs/29.0.50/
    Loading loadup.el (source)...
    Dump mode: pdump
    Using load-path (/home/viz/lib/ports/emacs/lisp)
    Loading emacs-lisp/debug-early...
    Loading emacs-lisp/byte-run...
    Loading emacs-lisp/backquote...
    Loading subr...
    Loading keymap...
    Loading version...
    Loading widget...
    Loading custom...
    Loading emacs-lisp/map-ynp...
    Loading international/mule...
    Loading international/mule-conf...
    Loading env...
    Loading format...
    Loading bindings...
    Loading window...
    Loading files...
    Loading emacs-lisp/macroexp...
    Loading cus-face...
    Loading faces...
    Loading loaddefs.el (source)...
    Loading button...
    Loading emacs-lisp/cl-preloaded...
    Loading emacs-lisp/oclosure...
    Loading obarray...
    Loading abbrev...
    Loading help...
    Loading jka-cmpr-hook...
    Loading epa-hook...
    Loading international/mule-cmds...
    Loading case-table...
    Loading international/charprop.el (source)...
    Loading international/characters...
    Loading international/charscript...
    Loading international/emoji-zwj...
    Loading composite...
    Loading language/chinese...
    Loading language/cyrillic...
    Loading language/indian...

    Error: void-variable (tamil--vowel-signs)
    (require cl-print) while preparing to dump
    make[1]: *** [Makefile:639: emacs.pdmp] Error 255
    make[1]: Leaving directory '/home/viz/lib/ports/emacs/src'
    make: *** [Makefile:469: src] Error 2

Should I stick in defvar's and declare-function's?