We have set this up in our instance of App Portal. While the built-in dynamic "Manager" approval works great, we do have some hardware items that, because of there type and\or costs, we want to "go up the chain a little higher" with in-regards to approval. The only way I found to do this, was to enable the "Custom Approver" feature. This allowed me to write a SQL statement that queries users by specific words in their AD "Title". Basically I included all Directors and VPs. Once attached to a workflow, the requesters are asked to select their "approver" from the list (which includes the users from my SQL statement). Worked out pretty well, but as Jason mentioned, there is really no way to really enforce that the requester select the correct approver in their chain-of-command. Hasn't been a problem to-date though. One downside to this also is, as I understand it, you can only create a single "Custom Approver" query. So if it works well for you and you'd like to duplicate your efforts to expand to other custom groups, you are out-of-luck.
All that being said, I just finished upgrading our App Portal from 2013 R2 to 2015 R2, and this is one function that "broke" once upgraded. When attempting to open the list of approvers, there is no longer anyone listed. I've tried different steps to resolve but have so far been unsuccessful. I will probably be opening a Flexera support case for this Monday morning.
