How can I avoid TeX environment's when token filtering in LuaLaTeX?

Question

This is a follow-up to: How can I exclude TeX macros when counting a string's characters in Lua?

Thanks to @Henri Menke's excellent answer to that question (below), I know how to filter out macros like \textit{abc} with Lua. However, this approach below seems to break down with environments like \begin{quote}...\end{quote}.

\documentclass{article}
\usepackage{luacode}
\begin{luacode}
local function rndstring()
    local toks = token.scan_toks()
for n, t in ipairs(toks) do
    if t.cmdname == "letter" then
        -- random number from printable ASCII range
        local r = math.random(33, 126)
        -- create new token with that character and catcode 12
        local letter = token.create(r, 12)
        -- replace old token
        toks[n] = letter
    end
end
token.put_next(toks)
end
local lft = lua.get_functions_table()
lft[#lft + 1] = rndstring
token.set_lua("rndstring", #lft, "global")
\end{luacode}
\begin{document}
\ttfamily
\rndstring{This string works.}
\rndstring{\textit{This string works}}
\rndstring{\begin{quote}This String Does not work\end{quote}}
\end{document}

Answer 1

This sets a boolean flag and only changes letters while it is true. if \begin or \end is seen the flag is set false until a } is seen so the environment name is not mangled.

\documentclass{article}
\usepackage{luacode}
\begin{luacode}
local function rndstring()
    local toks = token.scan_toks()
    local on = true
for n, t in ipairs(toks) do
    if t.csname == "begin" or t.csname == "end" then
       on = false
    end
   if not(on) and t.cmdname == "right_brace" then
     on = true
   end
    if on and t.cmdname == "letter" then
        -- random number from printable ASCII range
        local r = math.random(33, 126)
        -- create new token with that character and catcode 12
        local letter = token.create(r, 12)
        -- replace old token
        toks[n] = letter
    end
end
token.put_next(toks)
end
local lft = lua.get_functions_table()
lft[#lft + 1] = rndstring
token.set_lua("rndstring", #lft, "global")
\end{luacode}
\begin{document}
\ttfamily
\rndstring{This string works.}
\rndstring{\textit{This string works}}
\rndstring{\begin{quote}This String Does not work\end{quote}}
\end{document}