How to isolate I2C bus when Master is being programmed?
You have the right idea, but I think your circuit can be simplified. 

In the idle condition both SCL and SDA should be high, so connect R1 and R2 directly to +3.3V. M1 and M2 have body diodes that must not be allowed to transfer logic low from the ATTiny to the MPU-6050, so you need to swap their Source and Drain connections.

The Gates of M1 and M2 can be driven directly from a GPIO pin, then M3 and R7 are not required. C1 and R5 now connect to Ground instead of +3.3V, and the GPIO pin pulls high to enable the I2C connection. 

An I2C 'Start' condition is defined as SDA going from high to low while SCL is high. A slave device shouldn't drive SCL unless it is busy and wants to slow down the data transfer, so SCL should be safe from interference unless the MPU-6050 sees a Start condition while it is busy doing something else. With SDA held high (by R2 with M2 switched off) the MPU-6050 won't see a Start condition so it should leave SCL alone, and M1 and R3 are not required.        







simulate this circuit &ndash; Schematic created using CircuitLab