Just a data point: I made a reservation at the Kimpton Palomar in Washington DC for $229. I saw a room on cancelon.com for $126. The claim was rejected, first because there was a "cyber special" rate of $189 so I did not book the lowest available rate; and second, because cancelon rates are not eligible for BRG.
I did some searching and did not see anything about how IHG/Kimpton BRG works with cancelon, so there's now a data point that it will be rejected. FYI.