If you had taken the time to do your background reading, you'd have found that proprietors around Muroran or wherever the hell he is from are tired of putting up with the bullshit of drunken Russian sailors. THIS is where the current exclusionary "policies" of the onsen in question are derived. Drunken, annoying, disruptive russian sailors that run off business.
If this is the case, being allowed into the bath is a simple matter. (In fact, it is a simple matter in any case.) All you need to do is to buy a properly wrapped box of sweets, and take it with you to the owner of the onsen to pay your respects and say proper greetings. After you have proven that you are capable of behaving like civilized Japanese residents, the owners will not refuse yours or your family's entry into the onsen. Instead, Arudodebito has chosen to prove himself as annoying and disruptive as any drunken sailor or more so, assuring that gaijins to come after him will have an even harder time.
I hope he loses his case. But I hope even more that a sensible white gaijin would bear a gift and a proper greeting to the onsen owners while the trial is still in progress and get the "exempt" status right under his nose. That should show him how "Japanese" he really is.