Hi Julian,
I tried.. but no success(prolly because of my ineptitude in the arduino environment) so for now will simply dedicate a 328p as an I2c Slave(without DuinOS) to interrogate my onewire temp sensors and pwm interfaced Co2(which can speak I2C also) sensor and yes mostly likely the HS1101 humidity sensor as I depend on an rctime emulation for reading.
Then I will simply convey the results via I2C on the sanguino running DuinOS as I2C works just fine(duinos currently using I2C to read the 1307 RTC) (fortunately I have a few arduino capable boards in my junkbox).
As I will get home over a local holiday this week I might be able to find some DS18S20(not too accurate but speaks one wire just fine) and mail 2 or 3 out to you(they are kind of small) and see if you can make anymore progress than I have(if you are interested)?
gwen