If I didn't have a logic probe, then I'd use a small microcontroller, perhaps an ATtiny. (For quick checks, I use one of my microcontroller dev boards.)
If you want the indicator to have minimal effect on the I²C bus, then you need some kind of a buffer. I might try a 74LVC2G17, which is dual Schmitt trigger with 5V tolerant inputs; the input currents are about a microamp, so insignificant considering the typical recommended I²C pull-ups of 1k-10k.
All in all, I'd expect the components for this, including the microcontroller, to cost about USD $1, definitely under $2, at Mouser in singles.