Hi - I don't have an AR750, but on the A1300 (Slate Plus, also not DFS certified) with 4.5.x firmware you can Join 5GHz networks on DFS channels as a Station using LuCi by following the process HERE . You won't be able to Repeat on the 5GHz DFS channel but the 2.4GHz AP will still work for downstream devices. There's probably a way to script this, but for me using LuCi is simple and works. Hope it solves your problem...
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