Okay, this one is my fault. I completely missed this when I was doing my work.
Sigil of Haunted Dreams is currently modeled as having a 100% chance on use of BS or HS to add 173 crit rating for 10 seconds.
According to wowhead, the proc only has a 15% chance to occur. No cooldown is mentioned there but it may have the standard 45s internal cooldown.
Shazear, I'd write a patch for this but I don't know how this would affect the sigil code you wrote since it's general purpose and I'd probably just end up hard-coding in a definition like I did originally.
References:
http://www.wowhead.com/?spell=60826