To do any kind of custom UI stuff, you would need to use something like AppleScriptObjC to access the Cocoa API - regular AppleScript doesn't have any dialog customizations available, although you could perform a hack such as repeatedly showing a dialog that times out, for example:
Code:
set countdown to 20
set interval to 2
repeat with timer from countdown to 1 by -interval
if timer ≤ interval then set interval to timer
set theResult to display dialog "Should I do something?" & return & timer & " seconds left" buttons ¬
{"Yes", "No", "Maybe"} default button "Yes" with icon caution giving up after interval
if (button returned of theResult) is "Yes" then
say "Yes"
exit repeat
else if (button returned of theResult) is "Maybe" then
say "Maybe"
else if timer ≤ interval or (button returned of theResult) is "No" then
say "No"
exit repeat
end if
end repeat