Both Technic Pack and Tekkit include ComputerCraft, which gives you access to Turtles, one of the most wonderful automation tools.
A Turtle is a programmable block just like a computer, but it has an inventory and is capable of movement, digging (when upgraded to be a Mining Turtle), block placing, and sensing its environment and inspecting the contents of its inventory. You might already be familiar with the Mining Turtle's use with macro commands –
tunnel being popular – but since it's a full-fledged a CC computer block you can program it with Lua directly. Giving it a program with a combination of looping, movement, and block placement commands, you can have a Turtle build arbitrarily complex structures.
If you're really ambitious, you could build Wireless Turtles and coordinate them via Rednet from a single program running on a central computer.
(As with most automation, Turtles don't like being unloaded as part of a distant chunk. The usual solution of World Anchors or Teleport Tethers can be useful for long-running construction programs.)