Reply for your second suggestion:
Although maybe not as accurate as being a built in feature, you could pretty much get the same effect by using the [pos] argument in /splay, and then using a timer, eg:
alias splayx {
splay $eval($ $+ 1- $+ $calc($0 - 1),2)
.timer -m 1 $calc($eval($ $+ $0,2) - $eval($ $+ $calc($0 - 1),2)) splay stop
}
/splayx <file> <starting position> <ending position>