Looks like ALEX rims are serious with the 650B market. Alex will be introducing their XD-Lite Rim in a 650B format very soon. These rims should retail for around $40, each. They will be disc specific, and eyeleted (stainless steel to boot). Weight on the 26" is 438g, so the 650B should be around 480g. 2.5" Max tire width.
BTI part #AE-6275