I think running the line underground is a dandy idea but there must be some provision for draining the water from it where it comes into the shop. If the line reaches the shop at a low point and then must rise up to separator/regulator/outlet this could cause a problem so a drain must located at the lowest point. Also be careful not to allow the line to sag anywhere along it's length which could easily happen even with black or galvanized when using lengths like that. A low spot along the length of the pipe can be especially troublesome since it will allow water to collect that can not be drained, this water will be expelled in spurts when it reaches a certain level in the pipe. What is so annoying about this problem is that the air may seem to be dry most of the time but as the water collects in the low spot it will restrict the air flow until it reaches a point at which it will pick up most of the water and shoot it out of the line almost like a water hose for a few seconds. After this happens all may seem ok for a while but the cycle will start all over again and you never know when the water will come shooting out!
Of course this need not be a problem at all since it is simply a matter of keeping that line straight and keeping the slope fairly constant.
BTW, there is arguments both ways as to which direction to slope the pipe but IMO either direction will be just fine as long as the drain can be placed on the low end. In all probability the slope here is going to be determined by the location of the compressor to the shop so just slope it whatever direction is convenient, just place that drain at the lowest point.