The question sums it up. How are you managing to identify similar subject assets without a DAM and metatags?
Hashtags? Multi-select field? Other?
In order of simple to best, I'd suggest:
If a WF project results in tangible assets destined for a DAM, the company assigns material numbers for us to use, which we then use to track the assets. However, the actual file names can be kinda loosely named while in WF because as long as they are in the project the name doesn't matter much.
If the asset is destined for the DAM, we have a custom-created tool to track project metadata (outside of WF) that is ilnked to our DAM to pull metadata at the time of upload.
The problem is a typical project is modifying multiple assets and WF does not deal well with spreadsheets or relational database lists well; we tried, it's clunky and alot of busywork.
We have a DAM as well as BOX.com but I highly second a lot of what Doug mentioned. I have several document reports that utilize prompts based on the project custom forms to help people find things within Workfront. The reports are only as good as the data surrounding it so if someone doesn't fill in the custom field than obviously that file won't be listed.