Unless you have a real reason to have a volt meter I'd let it go. Unlike the previous Gens there is very little space behind any of the instrument panels. There is a lot of space under the right side panel from the battery back toward the seat and some space under the panels in front of the storage box. The curvature of the panels may make mounting the volt meter awkward.
Pull the panels and have a look for yourself.
There will be limited clearance under any panel, even if someone told you they put a meter between the storage box and the rear edge of the panel around the box you will still have to pull the panel and check for interferences. The exact location for the meter will be tricky and you can't simply copy someone else's install without determining the exact spot to put the meter on your FJR.
I would highly recommend that you use an ignition switched relay to read power directly off of the battery posts.
I use the built in volt meter on my radar detector, it's not a Datel for accuracy, nor is it directly connected to the battery but it does make an adequate trend indicator.