I know that I can play a b2 Dorian scale over a DomSus chord with a lowered ninth, but what about over a major7 b2?
You probably won't encounter this chord very often, but you could derive a synthetic scale for it. A synthetic scale is made by altering a traditional scale to fit your purposes.
In this case, you could play a major b2 scale. For Cmaj7b2:
C Db E F G A B C
b9 is not an appropriate chord for a maj7 because the b9 note confuses the root of the chord. In any case, the general rule of thumb is simple: when you alter a tone, you simply keep the original underlying scale, while making the adjustment for that single tone.