that there is still new muslimgauze material being released now, 15yrs after his death, just blows my mind. he was apparently sending a new DAT tape to staalplaat and/or soleilmoon every single week at one point though. his family definitely receives all his royalties for most if not everything released.
as mikko said, as long as it's done with respect and contact is made with the artist's family, or people and/or acts close to them to discuss and arrange it, then yes, but every time what the artist would probably have wanted should still be considered and not took advantage of because they are no longer here to say, i.e would they want this recording released (if it was unreleased), or would they want this old tape or whatever reissued, etc.