I don't have OnAlert to try this, but if you do and your MS10A motion detectors are basically unusable due to false alarms, then here's a suggestion you might want to try.
Assuming you have enough of them, then you can double up and use 2 to cover each zone (both set to mode 1 - single motion).
Do NOT register the MS10A's with the DS7000 base.
In AHP/OnAlert, create a macro for each that will set a flag, delay a short time (like 15 seconds), then clear the flag.
Each of the MS10A's macros would first check if the other's flag is set.
If so, then both detected motion and the alarm can be triggered.
If not, then it is most likely a false detection.
The probability of 2 MS10A's false detecting within a few seconds of each other will most likely be very small.
Another possibility (similar to above) would be to just use one MS10A in mode 1 (single movement detection) and use AHP/OnAlert macros to simulate mode 2 (two detections) before triggering an alarm.
Current OnAlert users can verify if this is even possible.