os.execute doesn't work if called by event.timer?

[ItsTheKais 0]

I'm putting together an autorun for a program that lives on a floppy. It started like this:

local fs = require "filesystem"
local os = require "os"

local proxy = ...
fs.mount(proxy, "/mst")
os.execute("/mst/portal.lua")

Unfortunately, this doesn't work; the first few lines from the program show up, and then the OpenOS banner suddenly tramples over it to present me the shell prompt, because that's obviously what I wanted to have happen when I ran os.execute.

This thread seems to describe exactly my problem, and I attempted the suggested solution like so:

local fs = require "filesystem"
local os = require "os"
local event = require "event"

local proxy = ...
fs.mount(proxy, "/mst")

event.timer(2, function ()
  os.execute("/mst/portal.lua")
end)

But that didn't work either; the OpenOS banner shows up, and a moment later, there are busy lights and working noises from the computer and floppy drive, as though it were doing what it was supposed to, but nothing actually happens on-screen.

Why doesn't this work? Should I be doing something differently? Is there a better way to tell OpenOS to stop helping?

(I also tried shell.execute, but that didn't seem to do anything different.)