Qalhat looks like a joke nomination but apparently there is more than the lone building at the side of the highway. Unfortunately I can't report better news than the other visitors and my visit was in Dec 2018; you can “get” to the site in three ways:
• Snap a picture from the highway coming from the East. There seems to be a parking lot just for this purpose. If you have a good sense you can get a really close up view of the mosque standing there, or binoculars. Yes it is lame. Let's try number 2.
• Drive into Qalhat new town by following the misleading brown signs to the archeological site. Well, actually the signs stop after the highway turn-off but follow the road along the coast/beach and do not turn right into the town instead continuing onwards to the river where the road bends right and you can see the raised area beyond the river. Soon there is a gravel road turning left. No sign to indicate the archeological site is that way but soon a “CLOSED” sign greets you on the other side. For a brief moment you may think if you should drive up anyway but let me tell you there is a gate at the top and you will need to reserve downhill a gravel road. I didn't see any guards but this is probably out of the question. Still though, you could walk up the hill and see what's going on, just be warned that you may want to say you can't read English signs or something ;)
• #3. Remember the parking lot? There is a water runoff below it. Why should you care? There are steps going down past it all the way to the bottom and from there it's a 5-10min walk to the building. Roam free! Did I do that? I've done some naughty stuff to access sites (WHS and other) but I didn't want to get caught in Oman doing anything to offend them. Just saying this is an option. There is no sign to say it is off-limit from here so you can always play dumb.
So unfortunately this is one of those sites that are left closed for tourists and it looks like it will be like that for a long time. The mosque doesn't look special from the distance so it will have to be a low score until I one day see more of it and update the review. Wish I could review it better now, there are even pictures on Google map that show someone nearby either officially or naughty like I suggested. I am thinking that a guided visit would be something they could arrange from Muscat.