Hi,
Long before I have worked on Microblaze. There should be numbers of slots from processor peripheral bus or processor local bus, specially to processor peripheral bus you can connect pheripherals like your custom I2C (wirriten in HDL by you) by instanciating your microbleze system to your top level module of HDL. ) For this there must be option of invoking ISE from Edk of invoking Edk design from ISE.
If you find solution then please tell me. It will brush up my knowledge also
HTH.
Shitansh Vaghela