I’m confused, this seems to describe an everything bucket, which long-standing general consensus here holds is best done by ‘everything bucket’ apps, designed for the purpose, like Evernote and DEVONthink. I’m unclear as to the ROI either for developer or the wider user base of trying to turn Tinderbox into an ‘everything bucket’. What’s the gain.
Tinderbox is on a desktop OS (macOS in this case), for a good reason: many of the general affordances users want don’t call unduly on the power of today’s laptop or desktop computers. The inconvenient truth is the the same can’t be said for mobile devices.
I can one way to implement part of it and that is for File and URL types (others?), i.e. those with an action button in KAs or Get Info, could be allowed a custom icon (format? size?) per-attribute. Of course to be available anywhere, the artwork would need to be yet more file bloat in docs. More workable might be to allow custom artwork in an app support folder and leave it to the use to manage across multiple Macs as they see fit.
As @PaulWalters has noted, it would be perfectly possible to have a per resource prototype and manage difference that way, which seems a much more efficient route to go. Plus, as button cions for File/URL types are only shown in data tables that show the attribute name, does $EvernoteURL really need a different icon to $iThoughtsURL?