I think this is a valid request. However, I think a better way to handle custom fields is to consolidate them into a better keywords system. I have been working on such a keyword system for a while and hope to have something to contribute to the devel branch (which will become version 1.5). For the purposes of this feature request, you would set up a keyword category which would be your custom field name, then set up the keywords in this keyword category (your custom field values), and set up the interface to select it (using a config panel). In this case, you'd select for the interface a separate line item (to show up on the edit file page and the search file page) with a dropdown menu and whether a value is required or not.
Parts of this new keywords system are coded, parts are still only on paper. If you have any comments on the suitability of such a system for custom fields or suggestions for the interface, please say so. But remember that this board is a "no support" board, so you are merely suggesting and discussing a possible feature, and shouldn't expect answers to questions and shouldn't expect code post-haste. That being said, once I have such a system ready for public testing, having a user with a specific application ready to test is useful.