のようなマッピングを実現したかったのですが、

入力	結果
右 `⌘` + `A`	`1`
右 `⌘` + `S`	`2`
右 `⌘` + `D`	`3`
右 `⌘` + `F`	`4`
右 `⌘` + `G`	`5`
右 `⌘` + `H`	`6`
右 `⌘` + `J`	`7`
右 `⌘` + `K`	`8`
右 `⌘` + `L`	`9`
右 `⌘` + `;`	`0`
右 `⌘` + `:`	`-`

A question related to nested event · Issue #1128 · Hammerspoon/hammerspoon (追記: と、頂いた hetima さんからのコメント) を参考にのようにやったらできました。

rightCommandKeyCode = 54
rightCommandFlag    = 16
releasedFlag        = 256

local module = {}

local rightCommandHandler = function(e)
  local watchFor    = { ["a"] = "1", ["s"] = "2", ["d"] = "3", ["f"] = "4", ["g"] = "5", ["h"] = "6", ["j"] = "7", ["k"] = "8", ["l"] = "9", [";"] = "0", [":"] = "-" }
  local actualKey   = e:getCharacters(true)
  local replacement = watchFor[actualKey:lower()]

  if replacement then
    local isDown           = e:getType() == hs.eventtap.event.types.keyDown
    local replacementEvent = hs.eventtap.event.newKeyEvent({}, replacement, isDown)

    if isDown then
      replacementEvent:setProperty(hs.eventtap.event.properties.keyboardEventAutorepeat, e:getProperty(hs.eventtap.event.properties.keyboardEventAutorepeat))
    end

    return true, { replacementEvent }
  end
end

module.modifierListener = hs.eventtap.new({hs.eventtap.event.types.flagsChanged}, function(event)
  if event:getKeyCode() == rightCommandKeyCode then
    local eventData = event:getRawEventData().NSEventData
    if (eventData.modifierFlags & rightCommandFlag) == rightCommandFlag then
      module.keyListener = hs.eventtap.new({hs.eventtap.event.types.keyDown, hs.eventtap.event.types.keyUp}, rightCommandHandler):start()
    elseif module.keyListener and (eventData.modifierFlags & releasedFlag) == releasedFlag then
      module.keyListener:stop()
      module.keyListener = nil
    end
  end
end):start()

1 ~ 2 つの Modifier キーを同時押しした時の NSEventData.modifierFlags の値を地道に調べてみた

10進数バージョン

	左 control	左 shift	左 option	左 command	右 command	右 fn	右 shift	(なし)
左 control	-	393475	786721	1310985	1310993	8650753	393477	262401
左 shift	-	-	655650	1179914	1179922	8519682	131334	131330
左 option	-	-	-	1573160	1573168	8912928	655652	524576
左 command	-	-	-	-	1048856	9437192	1179916	1048840
右 command	-	-	-	-	-	9437200	1179924	1048848
右 fn	-	-	-	-	-	-	131332	8388608
右 shift	-	-	-	-	-	-	-	131332

2進数バージョン

	左 control	左 shift	左 option	左 command	右 command	右 fn	右 shift	(なし)
左 control	-	000001100000000100000011	000011000000000100100001	000101000000000100001001	000101000000000100010001	100001000000000000000001	000001100000000100000101	000001000000000100000001
左 shift	-	-	000010100000000100100010	000100100000000100001010	000100100000000100010010	100000100000000000000010	000000100000000100000110	000000100000000100000010
左 option	-	-	-	000110000000000100101000	000110000000000100110000	100010000000000000100000	000010100000000100100100	000010000000000100100000
左 command	-	-	-	-	000100000000000100011000	100100000000000000001000	000100100000000100001100	000100000000000100001000
右 command	-	-	-	-	-	100100000000000000010000	000100100000000100010100	000100000000000100010000
右 fn	-	-	-	-	-	-	000000100000000100000100	100000000000000000000000
右 shift	-	-	-	-	-	-	-	000000100000000100000100