If you haven't had any luck so far, perhaps you should consider a daughterboard approach, wherein you clamp the i/o's of a DS89C4x0 to 3.3 volts with a 74S1053 or two.

Those are 16-bit schottky-diode clamps to GND and Vcc (3.3 volts, in your case) that can prevent the signals from exceeding 3.3 volts by more than a forward-biased schottky junction, typically about 0.3 volts on the 'S1053,or going below that same voltage beneath GND, which shouldn't occur at all. If you use those clamps, the Maxim/Dallas part should still function normally, as the 3.3 volt inputs from the rest of your circuit should be "high" enough to be seen as ones by the MCU, particularly if you pull those inputs up to +5 volts with, say, 100K-ohm resistors. This should not impact the remainder of your circuitry, though you'll have to be the judge of that.

RE