I'm putting in a shower/tub unit and we want to have the tub spout on the side of the tub in the middle. The valve unit will be in the usual place on the end of the tub, with the shower head above it. The diverter is in the tub spout. Will this work, or would it screw up the diverter if we put it on the side?
A normal tub diverter valve works because when the tub spout is open, it requires less force for the water to come out of the spout than it does for water to get pushed up to the shower head. The water takes the path of least resistance. When the tub spout diverter is closed, the water is naturally forced up the pipe to the shower head.
The reason the instructions say to only use one elbow and to limit the distance is to reduce the resistance of the water flowing out of the spout. If the plumbing to the spout is too complex and provides too much resistance, you could have some water get pushed up to the shower head no matter what.
So, this plan might work, or there might just be too much resistance in the pipe snaking around the tub to the other side. It's hard to say for sure. If the valve is installed lower, I think you chances are better. Also the tub spout pipe will have to go around the tub and not over it because water would certainly flow out of the head all the time if you did that. Try to use only 3-4 fittings on the pipe - don't make any extra bends.
I looked around a little and found this forum post where a plumber used PEX pipe and three elbows even with the tub spout on the same side. That was enough to make water dribble out of the shower head. PEX could have made this problem worse because the fittings don't have a full 1/2" inside diameter so they restrict flow more than copper would. Still, doesn't look good for your plan since you'd also have a minimum of 3 elbows (with PEX, 4 elbows using copper).
Since you have the wall open you should consider replacing the shower/tub valve assembly to include the diverter in the valve assembly instead of in the spout. (Seems like from something you said that you are already familiar with).
I also think you should think really hard about moving the spout along the center of the long side of the tub. That is a nasty accident or injury waiting to happen.
From a plumbing standpoint it might work, but it would be very unusual. The water pipe would have to go through a number of studs. What functionality would this give you?
Also there can be electrical wiring in the long wall of the tub (to serve receptacles on the other side of the wall from the tub). It may not be best practice to have wiring in the vicinity of water pipes for a tub/shower, at least copper pipes.