If I recall right, you could add a pressure relief. Not sure how low they can be adjusted. I have seen some that use drop in cartridges. The bypass line would just T into the outlet side of the cooler.
I installed an inline cooler on a grape harvester. Oil from the cleaning fans go through a filter then to the cooler and back to the tank. I do not have a pressure relief, but there is a gauge on the filter housing to show pressure (usually stays at 0). And during grape harvest, we have encountered temps in the mid 20'sF some mornings/days. Cooler is big enough to handle the flow of cold oil, haven't seen the gauge twitch yet.
As long as the cooler dumps into the tank, it should be okay. The only other thing that might be an issue is, I'm assuming the hydraulic system is open center. I've seen some closed center systems that use two pump (charge pump and main pump). Dumping the oil into the tank starves the main pump as most charge pumps are lower output. Some of the older John Deere's used this set up. The main pump had more GPM then the transmission charge pump. Dumping the fluid to the tank starved the main pump. Deere solved this by putting a filter housing w/ a return line (so the oil in the closed center system was looping). The charge pump didn't have to work as hard (charge pump sent oil to the filter, then to the main pump), but it created a heat issue if running hydraulic motors. Just saying....
Hope it's the simpler system.....
Oh, by the way, that AKG cooler, it looks identical (maybe same manufacture, who knows) of one I bought to put on the grape harvester's transmission. It's a strange grape harvester.....bascially it's a Ford 3000 tractor on a high frame w/ the PTO running the main hydraulic pump. The OEM Select-O-Speed tranny has it's own oil supply...tends to get warm, so I'm planning to use the cooler on it (w/ thermostat to control the fan). OEM cooler is similar to an automotive set up, oil goes into bottom of coolant radiator. Haven't got to installing it yet.
