Boundary mics can work nicely in a grand piano. I have used
Crown PZMs with a hemispherical
pattern in pianos. You didn't say whether the piano is a grand or an upright, and whether you need
mono or stereo. I have never had good luck micing an upright on the inside. They sound boxy and artificial that way. From the back sound board is usually better on an upright. Did I mention that I HATE the sound of uprights?
For a grand piano,
gaffer taping the mic to the lid is the way to do it with a
boundary mic. Be careful to tape the cable so that it cannot come in contact with the strings. Placement changes the character of the sound a lot , so it's worth experimenting a
bit.
With one mic for
mono, I would try placing it about a
foot from the hammers, and centered on the keyboard, with the "front" of the mic toward the hammers (the 851 is a
cardioid). If you need isolation, close the lid, but the short stick would probably sound the better.
The 851 may not be the best choice because it has a pretty
broad, 10 dB high frequency boost, which could make it rather bright. Such a boost is nice for voice but not for most instruments. Another thing you can try is to use a reasonably
flat instrument mic on a straight stand. Put it right at the crook of the piano (the curve where it narrows), pointed toward the middle hammers. Prop the lid open just a couple of inches with a
block of wood, with the mic sticking inside. This will give a natural sound for jazz and acoustic music, but may not be close enough to the hammers for rock sound. An AT Pro-37 would be an inexpensive choice for this method.
If you do go with the 851, replace the
gaffer tape every show day. I once had a
PZM go plunk into the strings right in the middle of a show because the tape gave up after a few days. Not a good sound laying on strings. Do not use
duct tape, and be sure to remove all tape when you are done so as not to harm the piano.