In order to find the optimal Dragon Coin route for this potential category, I made a really terrible graph of the overworld:

Yellow nodes means the level has dragon coins. Since this map is a little hard to read, I also made one specifically for the Iso Level Tour. Now, in this category, we need 44 sets of Dragon Coins in order to unlock Miscellaneous Monument, which is Iso's last level. But let's pretend that restriction didn't exist for a bit. Without collecting DCs, the route would look like this (click for zoomable):

If this is still too hard to read, just look at the blue paths. Those are the areas we visit.

Now, in this route alone we actually pass through 27 nodes with DCs anyway, which means we might as well collect them in all those levels. Unfortunately, one of those is Misc Momument itself, which doesn't count since we need 44 BEFORE unlocking that level. But still, even 26 means we only need to pass through 18 more yellow nodes. In fact, we don't even need to complete every level we get DCs in, unless it unlocks another level we need to visit. (BTW, yes the other 3 in special world are worth it because one extra trip to the storehouse is a lot faster than 3 whole other levels.)

So all we have to do is go from each of the black rings in the picture above and visit as many new yellow nodes as we can without passing through any gray nodes. After doing this, the new route would be:

Except this gets us to 48 DCs (not including Misc Monument), which means we overcounted! So now the question is, which of these new levels we just added can we remove? Well to simplify this question, I drew each of these new branches in a smaller image:

Every node in this image has DCs in it. So all we have to do is start removing nodes from the right to shrink the branches until we're down to 44. So we could just remove the last 4 from the bottom branch, but we probably want to minimize the number of red nodes.

One more thing to mention is that, although unlocking Peaks of Revolution would require going through a gray node which isn't in a DC path, it would actually be pretty fast. Doing this would only cost about 40 seconds, so it might be worth it to unlock a new DC level. With this consideration, the new summary would look like this:

From here it's hard to tell what the optimal route would be using only a graph like this. We'd really have to know how long each exit takes, with and without DCs. I would assume the bottom branch generally contains longer levels, but obviously Disarray and Bridge Engineering take awhile as well. But anyway, this should help simplify whatever DC routing is left.