Update on the unexpected 5v pulses on the Onewire Data line:
Using a Mega (Data on Pin 10 connected via 4k7 pullup to 3.3v) , with Onewire Vdd reduced to 3.3v, the Mega reads the DS1820 OK, but the scope STILL shows spikes of 5v. This can't be coming from the OneWire (which isn't getting a sniff of any 5v), so it must be some sort of intermittent pullup within the Mega - which makes me more confident to try the same configuration on the Due - where any internal pullup would be to 3.3v.
There is also a confirming quote attributed to DS1820 manufacturer Dallas on this Raspberry Pi thread (http://93.93.128.176/forums/viewtopic.php?t=30359&p=266106):
Yes, you can make VDD = 5V and Vpullup = 3.3V. The Data line (DQ) is open-drain so it can only go as high as the Vpullup voltage.
But I'd still like to understand why I see those 5v pulses, if anyone can help.