I think Feliway is definitely applicable to cat spraying.
==================From the Cat Fanciers website:
Some cats start spraying in the house. You will want to first rule out any medical causes, such as FUS or cystitis.... Spraying is more often a behavior problem and urination is more often a medical problem. It is best to check with a vet first. If the problem is medical, then you will need to simply clean up the odor after the problem is treated, otherwise you will need to try some of the behavior modification outlined below (and you'll still need to clean up the odor). <snip>
Cats sometimes spray to mark their territory so sometimes an area for your cat that other animals cannot go to will help. Keeping the litterbox immaculate will help in other cases. <snip>
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The text above is from the Cat Fanciers website (link below) -- it has a ton of information and does include the following interesting tidbit -- spraying can occur when a "previously neutered cat has a bit of testicular or ovarian tissue remaining, possibly resulting in a low level of hormone which could trigger marking." <
http://www.fanciers.com/cat-faqs/behavior.shtml>