OpenServo and I2C Syncronization
A few possibilities come to mind:


Have an I2C Master device dedicated for each servo (a
microcontroller for example) and a single microcontroller dedicated
for generating a synchronization signal which will go to each of the I2C Masters.
If possible, configure all the servos to have the same I2C address. 
They will all receive the command at the same time thinking that it
is for them.  Absolutely will not work if each motor needs its own
parameters - only if they are to receive the exact same identical
commands.  Possible issues: multiple devices trying to drive the
line during ACK or during a reply may cause unpredictable bus
contention.  Might be worth a try though if they're all going to receive identical    commands.


A quick look at the OpenServo page tells me that these devices are re-programmable and you can write your own firmware.  It sounds like to me that plain I2C may not be the best choice here considering your synchronization problem and there may be an opportunity to do something clever by modifying the motor controller firmware to accept an additional 'sync' signal.

EDIT: Fake Name mentions a broadcast address in I2C and this may be hacked to your advantage. The official I2C Specification supports a "general call" address which is optional for the device.  Laid out in the spec for general call are: address change with reset, address change without reset, and software reset.  I suppose it is possible to modify the firmware to support a custom command and use(/abuse) the general call for your own purposes.