Yes, the 89B should block 50% light below 720nm however I found differences between the Cokin 89B and a screw on Hoya R72 with Maco 820c IR film. This, along with some manufacturers quoting different transmission thresholds (0% and 50%), is what confused me thinking the 89B and R72 were different yesterday.
Looking through both the Cokin 89B and the Hoya R72 at a light source you can see a difference in the amount of red transmitted. I suspect the Cokin differs to the standard 89B as the cutoff is more gradual - it may block 50% of light at 720nm but it looks to be transmitting more than 0% at 680nm.
Also I just had another look at the Lee filter (haven't used it yet) and it's halfway between an 88A and 87 as it begins transmission above 730nm instead of 720 and 740 respectively. 50% transmission is probably around 780nm which will make it black to SFX200 and some of the IR films.
Another option for Gary might be the Ilford SFX filter which is supposed to be close to an 89B and is available in a Cokin P size. Unfortunately Ilford are dropping SFX200 from their catalogue so it may not be available for much longer. Gary, what film will you be using this filter with and how important are transmission characteristics?