How can I patch my slab around a rough-in pipe in a difficult to access location?
You can mix a pourable patch for this repair, but I don't recommend it.  That's when you would experience bleed water.  You just wait for it to reabsorb after floating and before finishing. 

You would be better served with a moldable but not wet mix that is used for deck mud.  As such, it will pack and smooth down with a wood float, but won't run or bleed.

After floating, let it rest 10-15 min or until a thumb press has some resistance (as opposed to sinking).  Then use steel trowels for a smooth finish. 

Be sure to dampen the edges of the cut slab and spritz the gravel (check for an intact vapor barrier under the gravel, you might add one if missing).