You know what I think when it comes to the secret project?
I think Mark is very aware that he is getting older. He becomes 68 this year. Somehow I have the feeling that there is alot of material he still really wants to put on record before he isn't physically able anymore. I think his focus shifted more towards recording the things he feels the need for to get out there. Instead of putting out a few songs and then go on tour. I also expect the style is going to be a bit different than what he has put out so far. Maybe even jazz influences... who knows.
Besides that, imho, I don't find Mark really the type of person who would do/organize a tribute CD. He is a songwriter who wants to put out his own material. He might take part in a tribute project from another artist and put in his two cents by taking part in one or two songs, but that is about it.
Mark tributes artist in his own way, by writing a song about them. Examples: "Donegan's Gone", "Back To Tupelo" (tribute to Elvis), "Beryl", "Song For Sonny Liston", "Quality Shoe" (tribute to Roger Miller) etc.
Another reason why his next solo album takes longer are probably the many little projects he does in between for other artists. And it wouldn't suprise me he produced some albums for another artists in between, like Ruth Moody. Maybe even recorded another duet album. Little projects like this for example