Still sounds like his proposal is unlikely to get council construction approval.
An easement for a sewer pipe is not an easement for access. It might be possible to a road over a sewer inspection opening if it is the sort with a ‘man-hole’ cover.
Different sorts of service easements can have different things allowed or not allowed by whoever holds the title for the easement. Eg. where I am we have service easements through the site. All can have a footpath or asphalt over but it’s our problem if they ever need to dig up the pipes. You can’t have a wall parallel with the edge because if they ever have to dig up a sewer pipe it could fall into the trench. However, subject to some specifications you can have perpendicular wall or even one crossing over. We could put a carport roof over, but only if the roof is high enough for a digger to fit under. Footings have to be outside the ‘zone of influence’ of pipes – i.e. not putting a load on the pipe through the ground. However, if the easement has a water main in it, you can’t have any structure overhead. Lots of rules.
Large trees might be equivalent to your local council to what in the ACT would be a ‘regulated’ tree. Here, if more than 12m tall or more than some girth, the tree is automatically ‘regulated’ and permission is required for removal or even a damaging activity e.g. digging in the root zone. A good case has to be made to get permission.