From Reconstruction through the New Deal, Congress regularly used its power over the justices' pensions to incentivize or restrict judicial departures—and to otherwise exert leverage over the Court
Is Souter still hearing lower court cases? He regularly did so but have not heard of any in a while.
Did Kennedy or Breyer hear any lower court cases after leaving the Court? Again, I am not aware of any. I recall Breyer joining academia in some fashion.
One law professor thought Stevens never "really" retired & felt it unethical he proposed amendments and so forth.
Regarding the power of Congress to use pensions to motivate judicial retirement/etc I think that ship has sailed. They are certainly paid well but I suspect that any justice willing to trade the power and prestige of the court for cash could easily rake in far more just in speaking fees nowadays if they so choose not to mention what they could get if they became even a pro forma partner at some law firm.
Currently the justices tend to avoid this, in part because they are all fucking loaded, and also because they want the respect of seeming to have a sense of decorum etc.. which suggests that any pension bribery wouldn't work and that any big stick (retire or no pension, even if constitutional) could backfire.
Why can't you just say the justice appoints themselves to the position of retired justice? Supreme court judges can preside without the rest of the court in certain cases so they can wield the power of a court of law and thus congress can vest them with the power to appoint inferior officers.