The P90 'dogear' cover usually requires a screwhole in the side-flap and the shape is different, but there are lots of folks now making P90s for a humbucker route.
One is Pete Biltoft's HS90 or HS90A (Vintage Vibe Guitars). He made the blade pickup in my X150 and while darker (for jazz) it sounds fantastic. P90s are usually brighter and more defined, but if you call and tell him what you're looking for he'll make it for you. He even does custom winds, and his pups ship w 2 sets of magnets and you can easily change them out for additional tonal control. He explains it better than I can, but in my experience he's fast, not too expensive and very easy to deal with.
http://www.vintagevibeguitars.com/pickup_sc.html
Also, I believe Lindy Fralin and/or Jason Lollar both do custom winds and both have P90s that drop right in a hummer hole.