If you can get hold of this IC it could solve your problems.I played with them a few years back and they worked fine.
DESCRIPTION
The P82B96 is a bipolar IC which creates a non-latching,
bi-directional, logic interface between the normal I2C bus and a
range of other bus configurations. It can interface I2C bus logic
signals to similar busses having different voltage and current levels.
For example it can interface to the 350 􀀀A SMB bus, to 3.3 V logic
devices, and to 15 V levels and/or low impedance lines to improve
noise immunity on longer bus lengths.
It achieves this interface without any restrictions on the normal I2C
protocols or 100 kHz clock speed. The IC adds minimal loading to
the I2C node, and loadings of the new bus or remote I2C nodes are
not transmitted or transformed to the local node. Restrictions on the
number of I2C devices in a system, or the physical separation
between them, are virtually eliminated. Transmitting SDA/SCL
signals via balanced transmission lines (twisted pairs) or with
galvanic isolation (opto-coupling) is simple because separate
directional Tx and Rx signals are provided. The Tx and Rx signals
may be directly connected, without causing latching, to provide an
alternative bi-directional signal line with I2C properties.