RJD,
There isn't a canned report but to find customizations in an AX layer is to open the AOT, right click the AOT or node, and select Find... Change Search on the Name & Location tab to 'All nodes'. Click the Advanced tab and enter the layer you want to search on in the Layer field. Entering USR will retrieve all the USR objects. The results can be copied to be pasted into Excel for further analysis.
One thing to keep in mind that Find will only retrieve the highest layer if more than two layers modify the same object. For example, if the VendTrans table is modified by the VAR and USR layer, searching the tables for all VAR layer objects will not retrieve the VendTrans table because the USR layer is higher than the VAR layer. MS Support verified this is by design and not a bug.
Hope this helps.
Rick