well here is the latest on this topic
i still have not tried I2CWRITE and I2CREAD from PBP 2.6 with the SMBus yet but i have learned quite a bit
the address is 0
they didn't make it easy but found that in the spec (rather than the chip data sheet)

as mentioned the electrical thresholds are different - they are specific voltages rather than ratios of VDD, might be generally compatible but why mess around ?
i found that the PIC16F1508 specifically says i2c/SMBus on the first page and that there is a register bit to set the electrical properties to i2c or SMBus

my next concern was whether there would be a need for voltage translation and / or whether other i2c devices could co-exist on the same bus

since the bq20z70 operates at 2.5V i was concerned about the PIC host running at 3.3V or 5V and possibly having the bus Hi voltage dragged down by clamps or other issues, a close study showed
that there should be no issues whatsoever running the PIC at 3.3V or 5V and having other conventional devices on the same bus although i have not yet built hardware to test this

there is however a need for voltage translation when the Host is much lower than the SMBus or other i2c peripherals such as a 1.8V host (master) and slaves at 3.3 or 5V, NXP has chips for these situations

so my plan now is to use a 16F1508 or other chip with that specific feature and get someone to program the thing in C and switch the electrical type bit on the fly as required

i got the TI dev kit and withing an hour or three was able to commune with a smart battery sample, it was breathtaking to see the wealth of information available including individual cell voltages in the series stack