Thanks for bumping @Ivangayton, and for sharing, @Nishon! This came in just as we were deciding to take a first step in this direction for not the upcoming Collect release but the following one.
Our preliminary plan was to do something more along the lines of Add offline map layers without having to put files in Collect directory and Button to select an offline map file on the phone and move it at the right place. Having the user select a file rather than specifying a URL makes it possible to use device-to-device transfer through WiFi Direct, Bluetooth, etc.
We also discussed making it possible to specify an mbtiles file in form design so that Central would prompt for it and Collect would put it in the right place. @seewhy also proposed this in Send offline map to device from Central (we're not ignoring you, I promise).
We saw your post come in and haven't yet had a chance to consider the tradeoffs between that approach and others we've considered.
Are there special user needs that led you in the direction of specifying a URL in settings?