I would like to be able to use my external DVD player with my Nexus 7 tablet, is there a plug-in for that or a mount?
Yes it is possible if your DVD player has
USB output, something like this one.
If you have one, you have to use an
USB OTG cable (USB female to MicroUSB male) to connect your devices.
E.g.: As you can see in the video, it is working even without root access with a portable Samsung DVD player: Samsung Optical Smart Hub and a Nexus 7 with stock Android.
If you DVD player hasn't USB output I think there is a very little chance to connect them to each other directly.