Yes, you can. All Open Edition firmwares can be easily reverted to 1.50 through the Recovery menu. Of course, I can't imagine why, since Sony is, to day, unable to detect whether or not you're running a custom firmware (I mean, if they could, they surely would have locked us out of the most recent UMDs).
Besides, it's highly unlikely that Sony would go after individual users of OE firmwares. It's like the RIAA: their best targets are the people PRODUCING, DISTRIBUTING and SELLING content. Every once in a while they go after some small nobody just to make themselves feared, but you can't bring a civil suit against someone you can't find.