Sorry for my late response here - I'd say currently there is no (easy) way to use a different ROM of the SoftBank model. I wil require somebody to come up with a solution for that, but that may take time. I'll go to the SoftBank store on the weekend to see if they plan to come up with an English model.
Right now, the unit will work in Japanese and Japanese only, no way you can get it to work in English as far as I can say at the moment.
MMS also does not work, so you are restricted to SMS. I am myself contemplating taking it back (I made an arrangement with the SoftBank people last Saturday to try how far I can get...) and get me a unit form Singapore.
So, if you are not comfortable with the restrictions, I'd rather advise to wait with the purchase. Let's see how DoCoMo will handle this...