Thanks Demonwolf for this script!

alias rewind [
if (= (watchingdemo) 1) [
startmin = $arg1
add2alias mapstartonce [
gamespeed 1000
checkmin
]
demo $_demofile
] []
]


alias checkmin [
if (&& (= $chgspd 0) (= $minutesremaining $startmin)) [
chgspd = 1
gamespeed 100
echo (c 3)Rewind executed!
add2alias mapstartonce [chgspd = 0]
] []
sleep 0 [checkmin]
]

nice! :)