Filtering on Capacity Total and Capacity Used

Hey,

I first would like to thank you all guys for the terrific job you are doing with the Entity Registration module.

I am turning around an issue for a while, though. I need to filter a view listing my upcoming events, based on the difference between "capacity total" and "capacity used".

The idea is to retrieve all registrations in the "waiting list", whereas "capacity total" - "capacity used" is > 0 (quite necessary to reallocate canceled slots).

I tried several options so far, none of which working :
- Views php (not working for filtering in D7 as far as I know),
- the filter "Global: field comparison" (Views dev version) is not working, I think because "capacity total" and "capacity used" are not declared as filtering data (whereas they are shown in the list of data available in "field comparison" filter).
- filtering on Registration Status is not an option, as I use the waiting list option (filtering on "waiting list" status + "open" state in the same time would not retrieve the data I need).
- I also tried to get inspiration from https://drupal.org/project/registration_views and from https://drupal.org/node/1651142 but with no luck...

If any one had the same issue and found a solution, I would be glad to hear from you !

Best.