It's do-able.

Start the race (The Driver), and just let him go.  Get rid of your two-star
by driving north a block to that bribe on the corner.  Then drive just south
of the Malibu and drive around to shake any persistent cops until you lose
the one-star.  When Hilary finishes, he parks on the grass just south of the
Malibu.  If you're there and see him do it, the car does not vanish (but
Hilary does).

Now, the car's locked.  You can't get in -- yet.

Here's the trick: PUSH it to one of your garages (like the one just north of
Mercedes' place).  Use a car (gently), or just get out and push it on foot.
(Once you get it rolling, it rolls fairly easily.)  Push it into the garage
and walk away so that the garage closes.  Open the garage.  The car is
repaired (as usual) -- AND unlocked.

It has a funny combo of characteristics.  It takes window-damage if you
shoot it with the sniper rifle, but you can't get it to blow up with weapons
fire alone.  It's rocket-proof.  But -- it takes collision damage and
finally blows, just like any other car.

So, Diaz's Admiral is still the car to have.  Unless you roll it or hit a
tank, it's essentially indestructible.  You can raise holy hell in it and
keep on rolling.

But it's just plain fun to steal Hilary's car.

--cleve