This is doable via a shell script (+ pandoc):

  # 1. Put markdown on clipboard
  
  # 2. Convert to hex
  out=$(pbpaste | pandoc -f markdown -t html -s | hexdump -ve '1/1 "%.2x"')
  
  # 3. Convert to an applescript data class
  osascript -e "set the clipboard to «data HTML${out}»"

  # 4. Paste rich HTML

You can use something like Alfred or Keyboard Maestro to trigger that script with a keyboard shortcut.