I'd imagine that as a visual sign of reperations after the Clone Wars the Seperatist droids would have been re-programmed to work on reconstruction. I could also imagine that civilians of former Seperatist world would be used a slave labour along those lines too.

The Death Star being (as of the PT) a former Seperatist weapons project may have used Geonosian and Neimodian tech and machinery in it's construction (a bit like Project Paperclip).

Assuming that the same machines used to build the first Death Star built the second there could be a long legacy of PT tech in the OT era.

I can't see a place where it would fit into ESB:R though.