Thank you, Skrommel.

It works and I like it.
I made a very basic minute countdown-timer version, which probably needs neatening up, and which would be much more useful with hours, minutes and seconds as input than just minutes.
InputBox, TimerValue, Timer Value, Minutes to countdown:
AlarmTime =
AlarmTime += TimerValue, minutes
alarm=%AlarmTime% ;Alarm time
height=2 ;Progress bar height >0
message=Time's up! ;Message to display when timer reached
#NoEnv
#SingleInstance,Force
range:=AlarmTime
EnvSub,range,%A_now%,seconds
Gui,+ToolWindow +AlwaysOnTop -Caption
Gui,Add,Progress,% "Vprogress X-2 Y-2 W" A_ScreenWidth+4 " H" height+5 " Range0-" range
Gui,Show,% "X0 Y0 W" A_ScreenWidth " H" height
Gui,+LastFound
guiid:=WinExist("A")
WinSet,Transparent,150,ahk_id %guiid% ; Transparency
WinSet,ExStyle,+0x20,ahk_id %guiid% ; Click-through
Loop,%range%
{
Sleep,1000
GuiControl,,progress,% range-A_Index
}
If message<>
MsgBox,%message%
ExitApp
I was trying to work out how I might use a tooltip to see what the progress was, but I couldn't make it work at all. Could it work? Could a tooltip show up displaying time left/total time when the mouse hovers over the progress bar?
And is there any way to have more than one of them running at the same time? I imagine they'd have to show underneath each other in the same toolwindow.