There are some mods on modDB, that are made for specific server and wont work anywhere else.
I also want to make a small tweak-collection mod without any theme or purpose for my local server where I play with friends. It would be cool, if I upload it on modDM and mark as not displayed, so when someone connecting, it will be downloaded automatically, however it wouldn't be visible in normal browsing.
As an alternate solution -- make it possible to download such private mods directly from server. This could cause security issues, so there should be a warning in client I suppose, but the possibility would be great.