Customising collection rules

[jackmaker]

Hello, 

Has anyone had any experience with customising collection rules? I need our 'Sale' collection to populate from the rule 'Compare-at price' is greater than 'Price'. 

The options at the moment are only to show if the field is empty/not empty or less than or greater than a specific value. 

The reason for this, is I need the compare-at price to be populated in all products so that our pricing app can function correctly. Currently it can only change the price field so if something is being reduced from being at full RRP and not having a value in the compare-at price field, it will show as the reduced price but with no strike through/full price shown.

Thank you in advance!