It’s probably ok. The entire 5V rail can supply up to 2 amp. OpenSprinkler’s ESP8266 draws about 100 to 150 mA (if you use wired Ethernet module, that’s another 180mA). So it can easily support another gadget that needs 200mA. Keep in mind that all this power eventually come from the external 24VAC power adapter (if using AC) or 7.5VDC power adapter (if using DC), so if you also want to run many zones simultaneously then it’s going to strain the external power adapter. But if you don’t run more than 1 or 2 zones at the same time, it should be ok.