Use the following hack ..
Links and Downloads Manager 2.1.0
https://vborg.vbsupport.ru/showthread.php?t=91107
You can have the files on downloads.com linked using LDM .. have a linked forum from forums.com/hidden (the hack does that for you) .. and the links/downloads can have the permissions from forums.com/hidden ..
The hack can hide the URL of the files stored on downloads.com .. so even members of usergroup100 wont know the real URLs and even if they post the URL of the download within LDM non members of usergroup100 wont have the permissions to use the URL ..
That the sort of thing you're after?