James Ghetty says:
When James enters a posse, place 4 ghost rock on him. The ghost rock may be used to pay for Shootout abilities (it is not a bounty). Discard his unused ghost rock after the shootout.
(Lillian has a similar clause)
Can this Rock be used to:
1) Return Accidental Reunion (IE: can it be used for effects that aren't strictly the cost of the ability)
2) Pay the 3 ghost rock on Serendipitous Arrival (IE: same as 1, but also which of these end of shootout triggers happen first)
3) Pay the cost of an abomination brought into play by Clown Carriage (IE: pay costs that aren't directly on the printed ability)