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:
Return Accidental Reunion (IE: can it be used for effects that aren’t strictly the cost of the ability)
Pay the 3 ghost rock on Serendipitous Arrival (IE: same as 1, but also which of these end of shootout triggers happen first)
Pay the cost of an abomination brought into play by Clown Carriage (IE: pay costs that aren’t directly on the printed ability)
This was ruled on in the following thread:
As referenced in that thread, James Ghetty cannot be used for the effect for Accidental Reunion
Ghetty can be used to pay the initial cost for the Dude at the beginning of Serendipitous Arrival, but not for the 3 ghost rock at the end of the shootout.
Ghetty can be used with the Clown Carriage, as playing the Abomination is part of the cost.
Updated for clarity/ruling.
If it’s the Ghost Rock cost printed on the action card, Ghetty can pay it
If it’s a Ghost Rock cost before the colon in an ability, Ghetty can pay it; if it’s the first sentence (also a cost) of an ability, Ghetty can pay it.