I don't know of any way to get OnEnter|Leave for mouse disabled frames.
You might be able to fake it with checking region:
IsMouseOver() for your debuff icons.
(on a generous timer or similar workaround)
Edit2:
If I tried to do something like that I would probably use a timer frame with a relatively slow refresh
(say 0.2 that seems to be a favorite with Blizzard).
Then have the OnShow() for each debuff icon add the frame to a registry and OnHide() remove it.
Every 0.2 loop my registry and check if mouse is over an icon and show/hide tooltip accordingly.
If no frames, hide my timer frame.
Not sure about processing cost but I don't think it would be too high.