Of course you don't mount files, you mount block devices, so I assume you mean you want to mount disk images stored on an nfs-shared filesystem? Do you want the nfs share to be always mounted and only mount the loopback images when accessed? Or do you want to have nfs+image mounted when accessed?
In any case, autofs can probably do the job. With it, you simply associate a mount command with a certain directory, and whenever someone tries to access it autofs will automatically run the mount, then time out and unmount after a user-defined number of seconds of inactivity.
Here's a page
detailing how to configure autofs, including how to mount network shares and scripting more complicated actions. I can assume it's possible to configure it to do what you want, but you might have to experiment if it's really complicated.
I should note that I had one problem back when I was using autofs to mount my smb shares with my old Windows computer. Sometimes when I tried to move very large files autofs would want to unmount in the middle of the data transfer, often ruining the copy and locking things up. It's been a few years since I've used it though, so don't know if that's still a problem with newer versions. I may just have been setting my timeout setting too low.