if i recall correctly, all you have to do is xacto off some of the plastic label on your AA batteries and they'll mount fine in an unmodified unit.
and i also think they were doing it for a flat rate of $50 at one point.
I don't really recommend trying to peel away the rechargeable battery jacket. I did that when I was trying to see how to get it to work, and when I turned on the MP-2 there was some weird, loud, electrical-sounding "sizzle" and I instantly turned it off and got rid of that battery. Don't think it's recommended to mess with the sleeves as they probably serve some purpose (googling perhaps would turn something up).
FWIW, flat rate for MP-2/MixPre servicing nowadays is $75 plus parts and shipping to/from SD. Costs about $100 if you don't need any major parts.