The most relevant thread I could find was: http://forums.netdui...-onewire-alpha/ In reply #145 (21 Feb 2013 pg. 8) CW2 states:
OneWire implementation for .NET MF 4.2 with the necessary modifications and fixes to make it work on Netduino/Mini/Plus (Gen. 1 boards) is now being evaluated and possibly integrated into the official firmware by Secret Labs.
It now seems a decision was made not to include OneWire support in NETMF 4.2 for the Netduino Mini.
Apart from native code, I'm not clear on what options there are for using a Mini to read OneWire devices.