You could try this macro but I don't know if it's what you want.
#include "slick.sh"
void _workspace_opened_set_backup_dir()
{
set_env('VSLICKBACKUP', strip_filename(_workspace_filename, 'N'));
}
This sets the backup folder path to be the same as the location of the workspace file and is executed when you open a workspace. It doesn't put all your backup files in a single folder - that would create name clashes. Instead it creates a folder tree rooted in the specified folder. If you want to try it, add the above code to vusrmac.e (in your configuration folder) and load it using the load module command on the macro menu. To "unload" it, delete (or comment) the above code and reload vusrmac.e.
If you re-locate the project, your "backup folder tree" will become invalid and you may need to copy part of it manually.
Graeme