Okay, here is a patch that does what I want.

That is, when the user does not select any desktop environments, it
prompts them about enabling NTP, with the checkbox already checked.

However, with this patch, the installer crashes after partitioning, like
this:

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In ./gnu/installer/services.scm:
     38:0 19 (%system-service-snippet-procedure (#<<system-service> name: "Network time service, to set the clock automatically" type: administration recommended?: #t snippet: ((service ntp-servi…>))
In ice-9/boot-9.scm:
  1669:16 18 (raise-exception _ #:continuable? _)
  1667:16 17 (raise-exception _ #:continuable? _)
  1667:16 16 (raise-exception _ #:continuable? _)
  1667:16 15 (raise-exception _ #:continuable? _)
  1667:16 14 (raise-exception _ #:continuable? _)
  1667:16 13 (raise-exception _ #:continuable? _)
  1667:16 12 (raise-exception _ #:continuable? _)
  1667:16 11 (raise-exception _ #:continuable? _)
  1667:16 10 (raise-exception _ #:continuable? _)
  1667:16  9 (raise-exception _ #:continuable? _)
  1667:16  8 (raise-exception _ #:continuable? _)
  1667:16  7 (raise-exception _ #:continuable? _)
  1764:13  6 (_ #<&compound-exception components: (#<&assertion-failure> #<&origin origin: "struct-vtable"> #<&message message: "Wrong type argument in position 1 (expecting struct): ~S"> #<&irrit…>)
In ice-9/eval.scm:
    619:8  5 (_ #(#(#<directory (guile-user) 7f8be12f4c80> #<<installer> name: newt init: #<procedure init ()> exit: #<procedure exit ()> exit-error: #<procedure exit-error (file key args)> f…>) …))
    619:8  4 (_ #(#(#(#<directory (guile-user) 7f8be12f4c80> #<<installer> name: newt init: #<procedure init ()> exit: #<procedure exit ()> exit-error: #<procedure exit-error (file key arg…>) …) #))
In ice-9/ports.scm:
   463:17  3 (call-with-output-file _ _ #:binary _ #:encoding _)
In ice-9/eval.scm:
    619:8  2 (_ #(#(#<directory (guile-user) 7f8be12f4c80> wrong-type-arg ("struct-vtable" "Wrong type argument in position 1 (expecting struct): ~S" ((#<<system-service> name: "Network …>)) …)) #))
    159:9  1 (_ #(#(#<directory (guile-user) 7f8be12f4c80> wrong-type-arg ("struct-vtable" "Wrong type argument in position 1 (expecting struct): ~S" ((#<<system-service> name: "Network …>)) …)) #))
In unknown file:
           0 (make-stack #t)
ice-9/eval.scm:159:9: In procedure struct-vtable: Wrong type argument in position 1 (expecting struct): (#<<system-service> name: "Network time service, to set the clock automatically" type: administration recommended?: #t snippet: ((service ntp-service-type)) packages: ()>)
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So, I've done something wrong. But I don't know what :)

I'm going to keep trying but hopefully somebody can help me.