asso = <|"x" -> {
    <|"a" -> 5, "b" -> "y", "c" -> {5, 6, 7}|>,
    <|"a" -> 6, "b" -> "z", "c" -> {}|>}
|>

asso // Query["x", Select[#c === {} &], Framed@*"b"]

{Framed["z"]}

asso // Query[
  {"x"},
  MapAt[
    MapAt[Framed, #, "b"] &,
    #,
    Position[#, _?(Function[asso, asso["c"] === {}])]] &
  ]

<|"x" -> {
 <|"a" -> 5, "b" -> "y", "c" -> {5, 6, 7}|>,
 <|"a" -> 6, "b" -> Framed["z"], "c" -> {}|>}
|>

bSel = MapAt[Framed, #, "b"] &;
cPos = Position[#, _?(#c === {} &)] &
cSel = MapAt[bSel, #, cPos[#]] &
cSel /@ asso

asso // Query[{"x"}, All, Query[If[#c === {}, {"b" -> Framed}, All]][#]&]

asso2 =
  <| "x" ->
       { <| "a" -> 5
          , "b" -> "y"
          , "c" -> {5, 6, 7}
          |>
       , <| "a" -> 6
          , "b" -> <| "x" ->
                        { <|"a" -> 5, "b" -> "y", "c" -> {5, 6, 7}|>
                        , <|"a" -> 6, "b" -> "z", "c" -> {}|>
                        }
                    |>
          , "c" -> {}
          |>
       }
   |>

asso2 //
  Query[{"x"},All,Query[If[#c==={},Query[{"b"->Query[{"x"},All,Query[If[#c==={},{"b"->Framed},All]][#]&]}][#]&,All]][#]&]

ReplacePart[
  asso2
, ii:{"x", i_, "b", "x", j_, "b"}
    /; asso2[["x", i, "c"]] === {}
    && asso2[["x", i, "b", "x", j, "c"]] === {}
    :> RuleCondition@Extract[asso2, ii, Framed]
]

drill[row_] :=
 Query[
   {"x"},
   All,
   Which[
     #["c"] != {}, #
     , Head@#["b"] === Association, Query[{"b" -> drill}]@#
     , True, Query[{"b" -> Framed}]@#
     ] &
   ]@row

drill@asso
drill@asso2 (* from WReach post *)

ClearAll[seekReplace]
seekReplace[row_, {}] := row
seekReplace[row_, cmd_?(VectorQ[#, AssociationQ] &)] :=
 Query[
   {"x"},
   All,
   Which[
     First[cmd]["ExitSeek"]@#, #
     , First[cmd]["Seek"]@#,
     Query[{First[cmd]["SeekKey"] -> (seekReplace[#, Rest@cmd] &)}]@#
     , True,
     Query[{First[cmd]["ReplaceKey"] -> First[cmd]["Replace"]}]@#
     ] &
   ]@row

seekCommands = <|
   "ExitSeek" -> (#["c"] != {} &),
   "Seek" -> (Head@#["b"] === Association &), "SeekKey" -> "b",
   "Replace" -> Framed, "ReplaceKey" -> "b"|>;
seekCommands = ConstantArray[seekCommands, 2]

seekReplace[asso, seekCommands]
seekReplace[asso2, seekCommands]