That was a custom order ES-1, with P-90 routs, no stop bar holes, and dot inlays not trapezoids, so I could make quite like an ES- 33. it was also supposed to have thin edge binding, but it came with the standard binding.

Yes, certainly possible to plug, then fill and sand and paint over the stop tailpiece holes and then fit a trapeze tailpiece. Same with the output jack. It's easy enough to fit one on the side. Just drill a hole I'd suggest one on a curved metal plate, rather than just through a hole in the side. It makes it a lot easier to fit and gives the jack more support with that edge being pretty curved.