I constantly see references to video being the application where an Optical Low Pass Filter is most useful, but it seems that still images do best without one. Would it be possible to then have a camera with a sensor sans the OLPF (such as the D7100) apply the benefits of an OLPF through a special filter on the lens? Do any manufacturers make such a filter?
In cameras without an Optical Low Pass Filter (OLPF) that are becoming more common, the high resolution of the sensor often approaches or even exceeds the resolving power of many of the lenses that will potentially be used with that camera. This means the limits of the lens itself provide the benefit of reducing moire. Reduction of moire, also known as aliasing, is the whole point of putting an OLPF, also sometimes called an anti-aliasing (AA) filter, in front of the sensor.
An OLPF works by refracting the light striking it into four paths. Since theses are very tightly spaced, this has the effect of slightly blurring the image focused on the sensor just behind the OLPF. You can't detect the four different images because the size of the spread produced by the OLPF is about the same size as a single pixel well, which is the smallest unit of detection the sensor is capable of.
There are at least these problems with trying to do this from the lens.
Most of these issues are less severe if the camera in question skips pixels to produce video at HD resolution. Since the pixel size is the same but only about every third or fourth pixel is contributing to each frame of video, the margin for error in terms of the precise amount of blur induced would be much greater. There are a few such products on the market aimed at improving the anti-aliasing performance of cameras that already have an OLPF, but they are camera (not lens) specific and tend to install in the camera's mirror box rather than attach to the lens. They usually hold the reflex mirror in the up position the entire time they are installed.
The problem with aliasing in video is to do with the method of downsampling the high resolution sensor image to produce a 1080p video frame (roughly 2 megapixels, a fraction of the resolution of modern image sensor), which uses line skipping (only reading every three or four lines from the sensor).
This is akin to scaling images using "nearest neighbour" resampling in Photoshop (or other editing program). The gaps between the pixels that are read out is much greater than the 1 pixel blur radius of the camera's AA filter.
However there are after market solutions for video aliasing, including lens mounted solutions as you suggest, though I imagine the eventual blur radius will be somewhat dependent on focal length.
A better solution is to get a filter from Mosaic Engineering that mounts behind the lens, which are also specially designed for DSLR video (to provide sufficient blur for sensors that use line skipping).