I just read that Seb plan to play it next year.
I just look at the senior tout website and Arno is eligible but maybe not enough famous ? Santoro play it despite not a great achievements compare to the others but his job seems to be a little bit playing these exho or tour
To be eligible to compete on the ATP Champions Tour, players must have been either a World No. 1 singles player during their competitive playing careers, a Grand Slam singles finalist or a singles player in a victorious Davis Cup team.
So for the senior tour I'm not sure but there is some exho with formers players during all year and the Grand slam.