Actually, I have quite a bit of experience with mic'ing harp, but most of it Celtic lap harp, a bit of a different animal. Of course, you can't put it on Harvey's piece of plywood, and you don't have to deal with the pedals. Close micing a concert harp will not work unless the piece doesn't involve using the pedals (some pieces don't) Finding out what the harp is like, and what the music is like in advance will cut your sound check time a hell of a lot. A little WD40 on the metal pedal linkages (not the wood) of the harp will go a long way toward quieting down the pedals. Actually, new style uses liquid graphite. Ask the harpist if there is any lubricating they can do to quiet the pedals, and explain the problem. They'll probably be willing to help. Of course you won't be putting any nassssty stuff on their pricey harp, because they will blow a gasket. Find out if the material that's being played makes extensive use of pedals, or whether it just has sharping levers.
Lap (folk) harp is easier, as it has no pedals, and can be treated a lot like a classical guitar, if it is nylon strung (most are). A few are bronze strung, and will be more reverberent, like a steel string guitar as opposed to a classic. I've had my best luck with a pair of Oktava MC012's X-Y in front and about at the top of the harp, backed off about 3 feet pointed down at the center of the soundboard. Not the *whole* soundboard, but the mid point between the highest and lowest note in the song being played. A third darker mic (Oktava MK319 in my case) is placed over the right shoulder (assuming rt. handed harpist) to pick up the deeper bass tones. See the hole in the back of the thing? That's the sound hole, like if a guitar had its sound hole cut in the back!
Remember that a folk player is called a harper and a classical player is called a harpist, like the difference between violin and fiddle. I'm sure your client will appreciate it if you ask questions up front that show you are considering the unique properties of this instrument. So the questions are: nylon or metal strung? Do you have pedals and will you use them? When you get them in the studio, check the highest and lowest note that will be used. For concert harp, boost highs by placing it on a hard surface, for folk harp, boost lows by using an LD condenser on the back of the body. Hope this helps.- Richie