What you should do is alter the tinyurl alias, so that it returns the tinyurl like an identifier: $tinyurl(<longurl>) and it returns the shorturl.
Then you can do something simple like this:
alias tinyurls {
var %a, %b = $regsub($1,/(?=(?:ftp:\/\/|https?:\/\/|www2?\.)[^<>\.\s]+(?:\.[^<>\.\s]+)+(?:\/[^<>\.\s]+)*)(\S{51,})/Sg,$tinyurl(\1),%a)
return %a
}
//echo -a $tinyurls(<string>)
This way, you can pass the entire sentence to $tinyurls, and it will return the same sentence with the longurls transformed to shorturls.
Since you've given no information whatsoever about how the tinyurl alias works, it might not even work, because we need the shorturls to be returned like an identifier. If this tinyurl thing works with sockets, then this won't work.