Yes, gotta love that the government are not the ones that discriminate against tattoos, but private organizations who should especially by now be happy snatch up all that tourist money are.
Technically speaking an onsen is a bath with special thermal ground water. Sento is a bath with regular water. So supersento is definitely a sento, it's just a private run one so they can operate by their own rules. Just look for public run sento, you can recognize them because they are very cheap, until 500 yen (subsidized, I guess). Private run ones would of course charge more.