There is no need for any complicated system. To build inside or outside of your border, you just need to place down the PW project where you want it to go. You can only rush inside your border. Project outside of your border will cost twice as much and take twice as long. This doesn't make expansion too fast. Those who's afraid of expansion rate should play CTP and get to know the PW system. In fact, it makes the expansion slower. Because every tile of improvement costs money. With workers, improvements are free, the only cost is the worker units themselves.
This won't cause expansion in modern ear either. You do have more money in modern time, but again, RR costs more than roads, and maglev costs more than RR.
I have played CTP and Civ3, and let me tell you, the PW system doesn't result in RR and roads everywhere because no one will want to waste their money building non essential roads. In civ3, you do have RR all over the place, and with infinite movement it transformed your whole country into some kinds of huge superconducting material, on which you units can magically teleport from one point to another in nanoseconds. If anything can be less realistic it is the RR in civ3.