I noticed Hybrid on the ITG series had tempo changes, and it was displayed as 185 BPM. However, setting a C-Mod in ITG2 cabs didn't invalidate the high score (which it would say "Will Disqualify for Ranking" in the options menu). This problem also occured in StepMania songs with a fixed DisplayBPM in the .sm file (not including the ones marked with *). This issue was on SM 3.9 final. Can you explain why?
It's by design. If someone fills out #DISPLAYBPM and makes it a single number instead of a range, it's because all of the BPM changes are minor and just there to correct the song sync. For that reason, C-mods won't DQ if the display BPM is constant.
Hybrid does have that slowdown at the end, so maybe it shouldn't have had a constant DISPLAYBPM. It's up to the person who creates the file though to judge whether the BPM changes are significant enough to disallow C-mods.