depends on what you consider a long time. You can't expect more that 3 hours from internal batteries in a MP2, which is fine if you are stealthing and don't want to deal with external batteries. I guess your mics may draw less phantom power than my ccm4 but I doubt it.
I used a 6v sla for mine, weighed a ton. If you are sure the mp2 can take the 9v, you can certainly modify the plug to the correct polarity and fit, if you are taping openly or an extended period of time I'd use the li-on.