The problem with this is that you are going to run into is that leasing requires an uninstall. You can not even enable leasing without having an uninstaller.
I have been pondering the problem, and I was thinking that perhaps you could use a software catalog item, rather than a general catalog item… Obviously, the software catalog item needs to target a package/application in SCCM.. Perhaps you could create a “dummy” package in SCCM, which would really do nothing (for example, a powershell script which simply ran an whoami command). You should flag the deployment as always rerun.. App Portal would submit an install/uninstall request to SCCM, but this would not really be doing anything. The important thing would be the actions tied to the 'on submit' for install and 'on submit' for uninstall. Ultimately, since the target package would not do anything, we would have a general catalog item which had both an install and an uninstall, thus we could enable leasing, and the associated install/uninstall actions. Never had anybody try this, but I can’t think of a reason why this would not work..
As I say, this is a little bit of thinking around the problem, and support for leasing non-software catalog items is not something Flexera has on it's roadmap, so you'll be on your own, but if you do try to do this, then please let us know, we'll do our best to try and resolve any issues you may have.