So Grinity, I think I get the spirit of what you're saying but not what you'd actually advise! The options are:

1) Don't bank on him getting him, so put him in another school entirely; if he tests in, move him (DD moved to attend the school, after all);
2) Put him in DD's school and hope for the best when testing rolls around in K;
3) Test him privately at age 5, maybe summer before he starts K,; put him in DD's school if he tests GT, or elsewhere if he does not.