A problem I've had with developing
SlashHelp is that changing the text in an editbox causes taint and until the editbox is cleared, secure commands can't be run from it. After discovering the default UI has its own tab handling for autocomplete, I left it to that.
I can look into adding in support for Lua code. I'll either have to manually provide a list of commands that support it as an argument or provide an API for third-party addons to register for.