Alarm systems and such just use a fixed voltage (ie 13.2v to 13.8v) from something really simple like an LM317. If you want to limit peak currents below the 1.8A current limit of the LM317 you can put a series resistor before the LM317. In your case with a 17Ah battery a 1.8A max current would be fine, and would only occur for a short time when charging after a long blackout period.
Sorry that does not help with the other point about stopping deep discharge, but if drawing only 350mA from a 17Ah battery it should be good for 40 hours of lighting or so, do you expect it to get worst than that? Normally emergency lighting is only used at night for 10-15 hours at max and the power would probably be back on the next day anyway.