I would gauge your players before I would take control of one of their PCs, even if the Spaltterman does a crafty job of hiding his tracks. Some players are good with having unique role-playing opportunities while others resent it when DMs take control of THEIR characters. Personally, I think this is a good idea since it raises the stakes to a personal level as far as dealing with the module's timer. The biggest problem I foresee is mechanics getting in the way of this happening, since the PCs get a Will save to avoid the temporary possession.
If you feel that your PCs wouldn't be cool with having control of their character usurped, consider using any of the other PCs they've encountered in Ravengro, particularly any that they've formed attachments to. Kendra is the most obvious candidate, but any of the council members or shopkeepers could work as well.
Perhaps you could have the Splatterman attempt to possess one or more of them through haunting dreams for a few nights, then have the dreams stop. If the players are clever, they will realize this means the Splatterman has found a new host.
I recommend the Sheriff, who might kill Gibbs in his cell for the blood for the next letter.