I've looked into this more and it is feasible.
The easiest way would be to have 10 choices, 1-5 skulls, 6-10 stars (rather than use negative -1 thru -5 numbers for skulls and positive 1 thru 5 numbers for stars).
Then add a couple of checks in the dorate scripts to determine how to display the rating, skulls or stars.
When I get some time I will see if I can make it work.
FC