The least creative and most time-intensive method would be to transplant shrubs into the clearing area. Many softwood shrubs, which are allowed to grow atop the pipeline, can grow rather large. However, to hope for the shrubs to grow to a size worthy of outright replacing the size of the now-removed trees is fantasy. For them to reach an even moderate height would take several years, and the clearing would look grossly empty while slowly waiting for the shrubs to fill in. Admittedly, this is the most likely option- but it might not be ideal.
Here are some rather crazy ideas that the park could do. They won't happen, but they are some creative options to "make lemonade out of lemons."
What about using thematic structures to obstruct the view? For instance, in San Marco, a building could be erected right in front of the area where the clearing meets the pathway. It wouldn't even have to be a functional or permanent building- it could simply be built of the same temporary materials that make up buildings like the Food & Wine Festival booths or prop buildings during Howl-O-Scream. The building could simply be decorated to look like any other building in San Marco, while really just being an elaborate prop. If placed directly in front of the head-on view of the clearing, it could obstruct views of the now-vacant land behind it, while fitting the area thematically. This may sound like a silly idea, but if this faux building was realistically crafted to look like San Marco's other buildings, it could blend harmoniously with the surrounding hamlet while not costing much more than a simple Howl-O-Scream prop.
Likewise, an option for the Wild Reserve is to set up a temporary animal display that is exactly the width of the pipeline clearing. Preferably, it would be an enclosure with height to it (like the rotating display near the cabanas) to effectively obstruct views beyond the enclosure. Some shrubs planted directly behind the enclosure, or even a simple ivy trellis, could prevent eyes from wandering beyond the animal exhibit.
Or, they could try to make the clearing look intentional. Since shrubs can still be planted above the pipeline, creating lush gardens on top of the clearing would make the space look deliberately used. In San Marco, for instance, they could design a garden that extends dozens of yards back into the depths of the clearing and has the same design as Da Vinci's Garden of Inventions. With similarly geometric designs, some statues, and maybe even a water feature, this "extension" of Da Vinci's garden would make the clearing look like a useful way of continuing the village's theme.
Similarly, on the Wild Reserve side of the park, colorful flower beds and small water features would draw attention to the clearing and make it look purposeful, rather than trying to hide it.
Another option? Pave over the clearing with aggregate. Take advantage of that now-cleared space. Expand the pathways onto the clearing. Sure, it would create a dead-end, but the new pathways could serve as space for useful items like games or food carts (and Food & Wine booths). Or even more simply: shaded seating areas. People are always looking for shady places to sit and relax, and a grove of tables and chairs surrounded on all sides by trees would be perfect. Plus, during special events like Howl-O-Scream or Christmas Town, extra path space is invaluable.
Again, these aren't what the park is going to do, or even what they should do. But damned if they aren't fun options to ponder.