While not condoning the practice, I have seen it done many times, and done it myself a few. In each instance, it would have been more dangerous to have a spotter, as the spotter would have been in more danger than the "overhanging" operator.
I firs seen this done on a deeeeep sewer. It was 32' to 35' cut to flow line. Digging was with a 1100 Hitachi long boom, long stick. This was max depth, and a shield was in use. The operator got the grade very close from the seat, then leaned out to make the final pass in the bottom, getting cut info from the spotter in the trench (only place he could be) This last pass was very slow and in control, and then pipe was lowered by crane, and digging started again for the next set.
This was over 15 years ago, before GPS machine control was even thought of. Now, on a job like this, there is technology to get it done safer, but not every contractor owns it, so people do what they must to get the job done.
I absolutely would not reccomend the practice, but a VERY experienced operator can do it. Another note, I would never do it on a Mini, or any machine much less than 20 ton. The small machines are much easier to jolt and cause a fall.
As Pete said, a camera would be a good choice, and they are getting better and less expensive all the time.
The comment above " My machine does not have a seat belt so I would not be breaking the law." shows me a lack of a lot of things, but mostly of concern for safety in general. Besides, the law does not say "if originally equipped" as it does when applied to motor vehicles. Also, you are describing a mini excavator, and they didn't exist before the seat belt laws.
I will continue to preach seat belt use. I have seen far to many accidents where ther is very little machine damage, and the operator died because he fell out and died from the fall or got crushed. I even put a seat belt on to go a short distance. Tracked machines especially can throw a person from the seat even over a small bump. Even if not thrown from the machine, it can cause head, neck, and back injuries. Stay in the seat with a seat belt!