I've tried covering it with a small box of the grass that I copied from another part of the image, but when it zooms in at around 0:27 it all goes to hell. I've tried keyframing, but it's all jumpy and jagged, prob because of the swooping motion of the original. Apart from keyframing every single frame, any ideas?
To smooth it out you need to track the area you want to remove. You can do this in AfterFX. Checkout this COW tutorial Camera-Tracking Part 1. There's also a part 2. I haven't watched this recently, but it should give you enough info so you can apply it to your project. CoreMelt has an plugin called Slice X that will do the job, but it's only for FCPX.
MacPro 8-core 2.8GHz, 16 GB RAM, OS 10.8.4, QT10.1, Kona 3, Dual Cinema 23, ATI Radeon HD 5870, 24" TV-Logic Monitor, ATTO ExpressSAS R380 RAID Adapter, PDE enclosure with 8-drive 6TB RAID 5
FCS 3 (FCP 7.0.3, Motion 4.0.3, Comp 3.5.3, DVDSP 4.2.2, Color 1.5.3)
FCP-X 10.0.9, Motion 5.0.7, Compressor 4.0.7
Pro Tools HD 10 w SYNC IO & 192 Digital I/O, Yamaha DM1000, Millennia Media HV-3C, Neumann U87, Schoeps Mk41 mics, Genelec DSP Monitors, Prima CDQ120 ISDN
My AE skills are as yet embryonic, but I'm sure John's idea will work. Because I'm stronger in Apple Motion than AE right now, what I would do is, first, make a new version of that entire lower right quadrant in Photoshop, using the edge of the road and the digital green as my boundaries. The warping control in photoshop is magical.
I wouldn't have to keyframe "the whole thing"; the beginning and end have long static parts in terms of the camera move, so you could just patch those with a still grab and clone brush to create a matte/mask over the letters. Don't try to move one little band-aid around on top of the letter; instead, re-make that entire "quadrant".
If I didn't want to mess with 3-d planar tracking, I would invoke the 3-d camera in Motion, bring in the base footage as one layer, and my photoshop still "patch" as another layer. The 3-d tool in Motion is easy to handle, and you can quickly "eyeball" the patch element into position for the beginning and ending sections. Turning on the recorder button and making only three or so keyframes at the middle and end points of the camera swoop, the machine will probably get the rest of it okay, with only a little adjustment to the curves.