You should already have that option.
As much for Saturn and Dreamcast. I have several games that have been translated into my language and i do not have the possibility to leave them with the covers.
Case of the Shenmue II of DC that was added voices in English.
The menu is horrible with games without covers.
I expect this option in the next MODE update.
I don't "expect" this feature, but it definitely would be a value add in my opinion. New games like "Xeno Crysis" for example isn't just a hack or mod, it's a "retail" game that was released on many platforms including current gen platforms, yet it doesn't have cover art via the official db_dc.bin.
I would love this feature more than anything. Just supporting something like "Folder.jpg" would be amazing. Also User images should override database covers, that way if a database cover is stuck in "cover loading" the user could replace it with their own cover.