I've recently been photographing with a 4.5mm circular fisheye lens and am finding that a square crop is a nice way to present the resulting images. Is there a way to indicate to Lightroom that I'd like the center of the square crop to be at a certain point in the image?
There are a couple of ways of centring a crop in Lightroom.
If you hold down Alt (Windows) / Option (Mac) as you drag a crop handle you get a symmetrical crop around the centre of the image.
To centre a crop on a specific point other than the centre of the image, go into Crop mode then keep pressing O to toggle between the various crop overlays. If you want to get near enough, the overlay featuring a pair of vertical and horizontal tramlines that make a small centre rectangle is good. If you want to be pixel-precise, keep going until you get the grid of squares. The centre lines are the ones that line up with the crop handles on the outside of the crop area.
Select the first picture and make your adjustments. Then click on Settings>Copy Settings. Make sure that Crop>Aspect Ratio is selected (you may also want Straighten Angle or any of the other stuff available, up to you).
Then select all of your other photos that you want cropped in the same way and then click Settings>Paste Settings. Depending on how many photos you have selected this may take seconds or minutes.
Unfortunately I did not see how you could make a preset with the crop. Nearly everything else present in the Copy Settings dialog is available for a preset, but not crop.