The P1900 is fully compatible with the Mac mini and does include a shutdown controller. The battery drain issue will be the same with any PSU solution for the Mac since it requires that the power continue to be supplied to the Mac when it is in sleep mode to maintain the system state in RAM. The problem occurs when you leave the Mac in sleep for extended periods of time. We won't know until we get some more hands-on experience how long the "extended" period is (days, weeks, etc). We do know that it will take 150mA to 200 mA to keep the RAM alive during sleep. The P1900 has a low battery sensor which will fully shut it down if the battery discharges below about 10.6 volts.
We're hoping some Mac experts can come up some sort of software solution/aid to this problem.
Hope this helps!