With luatexbase.add_to_callback() how can I conditionally get default behavior for open_read_file and find_read_file?

Question

Rationale: The goal I have is to create a system where I pretend -- via Lua -- that a file is being read, while in reality the contents come from elsewhere. A concept elsewhere called VFS (virtual file system), which is the name I borrowed.

Consider the following MWE (I know it has flaws):

% !TeX encoding = UTF-8
% !TEX TS-program = lualatex
\documentclass{article}
\usepackage{luacode}
\begin{luacode*}
do
    -- Table which contains "reader" functions for the virtual "files" given as keys
    vfs = { ["ZaphodBeeblebrox"] = function() return nil end }
local function luatexbase_log(text, ...)
    texio.write_nl("log", string.format(text, ...))
end

local function get_vfs_realname(name)
    local pfx = "virtual." -- virtual "file" prefix
    if name:find(pfx, 1, true) == 1 then
        local lookup_name = name:sub(#pfx + 1, #name)
        if vfs[lookup_name] ~= nil and type(vfs[lookup_name]) == "function" then
            return lookup_name
        end
    end
    return nil -- not found
end

local function hook_find_read_file(id, name)
    kpse_name = kpse.find_file(name)
    if kpse_name then
        return kpse_name
    end
    local rname = get_vfs_realname(name)
    if rname then
        luatexbase_log([[Found VFS \input lookup: %s (id=%d)]], rname, id)
        return name
    end
end

local function emulate_default_open_read_file(name)
    return {
        ["fhandle"] = assert(io.open(name,"r")),
        ["reader"] = function(self)
                return self.fhandle:read("*l")
            end,
        ["close"] = function(self)
                self.fhandle:close()
            end,
    }
end

local function hook_open_read_file(name)
    kpse_name = kpse.find_file(name)
    if kpse_name then
        return emulate_default_open_read_file(kpse_name)
    end
    local rname = get_vfs_realname(name)
    if rname then
        return { ["reader"] = vfs[rname] }
    end
end

luatexbase.add_to_callback("find_read_file", hook_find_read_file, "vfs")
luatexbase.add_to_callback("open_read_file", hook_open_read_file, "vfs")

end
\end{luacode*}
\begin{document}
\input{virtual.ZaphodBeeblebrox}
bla
\end{document}

As far as I can tell, the code works flawlessly. However, there's one thing I dislike: emulate_default_open_read_file().

I totally made up that function, trying to guess what LuaLaTeX does by default.

Question

1. Instead of coming with my own version of what I think LuaLaTeX may be doing by default upon "open_read_file", is there a way can access the default behavior? The goal here is to be 100% identical in behavior to the default, except for when I deal with my own virtual "files".
2. Assuming there is such a way above, how do I detect failure by the default function and kick in with my fallback? Or would the only way here to switch the order, detect the virtual "files" before and failing that fall back to the default?

---

NB: I am aware that the current "ZaphodBeeblebrox" reader function is flawed. It should return data line-wise and end with nil to signal end of file. But I got stuck before fleshing out this function. So please bear with me.

PS: I took the liberty to rewrite luatexbase_log from texdoc ltluatex to use the string.format() facility directly, which saves some typing. PPS: I am trying this in order to avoid having to write temporarily to the disk and in order to dodge the weird behavior I get when using \directlua or \luadirect to tex.print() a bunch of LaTeX+TikZ code. Probably related to expansion. But writing the output of the Lua code to disk (manually), then using \input on the written file, happens to work. Which is what I am trying to emulate here.