Only difference I could tell between a US and JPN Pioneer Laseractive was the text on the back of the unit for the connections etc, and the power cable itself seemed to be a bit smaller cable (physical diameter etc) Functionally, they are identical. The NEC unit looks slightly different cosmetically, and the only internal difference I found was one small add on PCB that was maybe 2" x 2". Not sure what it's exact function is, but as far as I can tell the two units operate identical.
And like Dan said... any PAC will work in ANY laseractive. And LD game will run on either region's PAC. Genesis carts and sega CD games require a US Sega PAC, MD carts and Mega CDs require a JPN Sega PAC. Though it might be noted that the region lock on Genny/MD carts is in the carts themselves and only on a handful of them. So most Genny/MD carts will run fine on either region's PAC.
Of course TG16/PCE pacs have the same limitations that the consoles do. The only region lock is on the hucard, which can in theory be overriden with a converter card (on US systems) or with a region switch... if you'd dare mod such a rare piece of hardware.
