Your query about observing locations in Zion NP has only drawn one response and very few hits so let me try to get the ball rolling with some comments and a few possible sites. I am sure someone down there has better information and hope they will chime in.
The caveats on my comments are that while I live near Denver I have been to Zion (and other parks in the area) several times in the past 10 years or so and have traveled through the area a number of times. I have stayed over night at Zion and in the area, but have not done any star gazing there.
If you have been to Zion I apologize for the following idiot lesson. The main area of the park, accessed from the south or east entrance, is a narrow, deep canyon running from the south-southwest to the north-northeast. While the road, Zion Canyon Scenic Drive, has some pull offs and picnic areas it is on the floor of the canyon and your horizon will be zenith down to maybe 45 degrees. The overlooks shown on maps are a 1,000 or more foot climb by foot from the road. So unless you are into transit viewing the main area is probably out. BTW: several years ago the Park Service closed the road in high season to private automobiles and took people into the canyon by bus, I assume that is still the policy, but not sure it will apply in April.
I have stayed in one of the three camp grounds at the south entrance/Springdale, but do not remember which one. The camp was good, but many old, large cottonwood trees formed a thick canopy. Some areas at the edges were exposed, but trees to the north and Watchman Mountain to the south and east limited the horizon. There are several amphitheaters near the camp grounds which might provide good views if the idiot light bombs have been removed.
I believe that an ideal location to observe from if it is open at night would be the Kolob Canyons Viewpoint, which is in the northwest section of the park, accessed from I-15. It is a fair drive from Springdale. The overlook is high with excellent views to the east and good to the west and north as I remember. There is a hill/part of the mountain to the south or southwest which should block light from that direction (important). I think there were a table and a vault toilet/porta-potty at the overlook. The road up the mountain twists a lot, but is good. This is a ‘lost’ part of the park with many fewer visitors than the main canyon.
The east entrance to the park had a pull off/parking area that might be an option. It is a good bit closer to Springdale than Kolob, but a winding mountain road through a long tunnel. Further to the east on route 9 toward Mt. Carmel there might also be some good spots, but the area is hilly/mountainous.
Outside the park the area is sparsely populated and quite dark except in the several very small towns. The big problem will be to the southwest where sprawling St. George and its environs put up a large light dome. I would think that any areas west and south of Virgin would have this problem.
North from Virgin is the Kolob road. The southern portion of the road is paved, but then turns to dirt and continues north to meet route 14, the Cedar City-Cedar Breaks road. There are probably a number of observing spots along the road, but lacking facilities. Notable locations are Lava Point, which is in the park and may have some facilities. As I remember the location it had sparse tree cover to the northwest and west and good views south and east. There is also a fire tower near here shown on the map; we did not go to it. Further north there are two reservoirs, Kolob Reservoir had a number of camp sites and some facilities. It was populated by a lot of fishermen when we drove through. The area was a wooded valley, but there were a number of side roads to barren hills.
As I remember the areas south of route 9 between Rockville and Virgin, there were a number of roads leading off to barren mesas/hills, there might be some good areas if you can block out St. George. North of rout 9 was a mountain(s).
Maps I have of the area say not to take dirt roads when wet.
The few years old Park Service map I have of Zion says ‘for information write: Superintendent, Zion National Park, Springdale, UT 84767-1099; call 345-772-3456; or visit www.nps.gov/zion on the internet.’
I hope this gives you some ideas to explore.
Clear skies with low horizons,