They really are 2 different things in almost every way. I believe that Dick Mills has this one right in that the best way to practice for throwing off a mound is to throw off a mound. I also agree with Steven Ellis that this doesn't mean that we should never throw on flat ground. There are certain things that are transferrable. You can work on arm action, firing the front hip at the target, rotation into landing, keeping the shoulders from rotating until landing, forward trunk flexion and landing with a flat back finish. Just be very aware that one of the most important aspects of pitching will be compromised, that being smooth timing of the parts and the "feel" and timing of it all.
Given that the landing spot on a mound is several inches lower than on flat ground, the timing of front foot turnover to land will be severely affected when doing it on flat ground. Also, you lose the energy gained from gravity if you throw from flat ground. On a mound, this adds to the total amount of momentum in the stride, which eventually can be transferred into the rest of the motion (assuming that the pitcher's mechanics allows that to happen). Release point can be affected which is something that is a matter of "feel" and can't be taught directly. That can only be done pitching in the environment you will be in during games.
So, having the same problem you do with availability of mounds to practice on, I have no other choice than to work on flat ground, do the best we can with what we have and get them on a mound as much as possible. Not working on mechanics at all is not an option.
Just be very aware that these are 2 distinctly different activities when it comes to putting the entire delivery together and that it has a significant effect on the results. Work on what you can and know what doesn't transfer.