Even if the coup had momentary success they would have been quickly overthrown. I sincerely doubt it, but we'll never know. Just reading the first three paragraphs of the
"Japanese Plan for September Defense of Kyushu, Report, Dec 1945." (PDF) lets you know they were more than ready and totally underestimated..
[snip]
"...it was estimated that 421,000 Japanese troops were on KYUSHU"
"Information secured since the occupation of reveals that the overall total strength on KYUSHU of Japanese units of all services and types as of the final day of hostilities totaled approximately 735,000."
Oh I have no doubt that they were ready for a land invasion. We would have been cut to pieces. But once the bomb made it's appearance we would not have even attempted a land invasion so the point is moot.
If the coup had been successful they would have been over thrown when the third or fourth bomb had landed. Remember that only half of the die hards that made up the war council were willing to fight to the death. Their base of support was very small indeed. It would have delayed the outcome but probably by no more then a few weeks at most.