Answering my own question, it looks like this works:
proc fseek(f: File, offset: clong, whence: int): int {.importc: "fseek",
header: "<stdio.h>", tags: [].}
var s1 = readAll(stdin)
discard fseek(stdin, 0, 0)
var s2 = readPasswordFromStdin("\ntest: ")
Anyone know if that would work on Windows?
