Many newer digital scopes have the ability to decode I2C natively, and can be set up to trigger on a variety of conditions. Shame about yours. (What scope are you using, by the way?)
If there's sufficient bus-idle time (SCL and SDA high) before the event, you could simply do a one-shot capture of the first edge of SDA going low, which would correspond to the start event. Make sure you have the trigger timebase near the left edge of the screen to capture maximum data.
If your scope can do a little more advanced triggering, you could look for the falling SDA edge while SCL is high, which would prevent false triggers on data transmission (i.e. not triggering when SCL is low and SDA changes, which is the norm other than for start/stop conditions).
Neither of these require the use of any external triggers or additional circuitry. You can use a D-latch to generate a start event trigger if you must go external - Q will go from low to high on a valid start condition. (I've not tested this, so YMMV)