It is ok to connect the probe as long as you can guarantee that the barrel of the BNC jack is connected to Neutral (never to Line). This is analogous to what I would do if I were using my Tektronics scope to measure line voltage at home; I'm fine as long as the probe tip goes to Line1 (Black wire), and the probe's ground clip goes to Neutral (White wire). God help me if the clip ever touches the Black wire! The scope frame is grounded through the Fat pin (Green wire) on it's own line cord. In the US distribution system, that effectively connects the scope frame to Neutral, meaning that the 10X probe ground clip is connected to Neutral, too.
The problem of using an opto isolator is that the transfer ratio is not that well defined. I would use a small plug-in AC transformer (12V?, without rectifier/capacitor). The turns-ratio is absolutely defined. At low currents, it will give you a good reduced representation of the input voltage. You also have complete isolation.