I'm doing some research on this, what I've been able to figure out is that typical Japanese road signs like "closed to traffic" and stop are reversed but I'm trying to figure out the meaning of the reversed signs.
通行可： Yes. You're correct. This is the opposite of 通行止. "You can go through here."
It depends on the situation where you can see this sign.
For example, when you encounter a road repairing, you may see 通行止 on the automobile road. However, you may see 通行可 on the pedestrian walking.
In this case, automobiles are not allowed to go through, but pedestrians are okay.
Likewise, 進め＝"Go!" or "You can go," "You may go" according to the context.
Therefore, I don't think there are special meaning on those signs. They literally mean it.
One thing I must add is that those signs are not the officially-defined Japanese traffic sings. They are someone's own creations, probably for their certain special purpose.