Considering signs don't last forever and the 100 year timeline that was given, I don't think the cost would be prohibitive. You could just update the signs during their normal replacement cycle. I'd imagine it would be something like this:
Years 0-25: All signs replaced must print both measurements
Years 25-50: All signs replaced must print both measurements, both with equal representation.
Years 50-75: All signs replaced must print both measures, with the metric measurements featured.
Years 75+: All signs replaced must print Metric only.
I guess some signs will need to be larger to accomodate more info, but probably not a "considerable" amoutn more then the normal cost.
The biggest problem with this proposal is that any legislation like this would just get repealed when the "other" political party takes office. There is almost no way that this would last for 100 years.