The problems with underpinners in these modern times is there are now a lot more hardwood mouldings than when those things were designed. Nailing those mouldings with fairly dainty underpinners and too-small v-nails can often leave something to be desired in terms of getting the nails in at all, much less stacking up the 2 or 3 nails you often need to cover the depth.
Another problem is that the butt joints used for frames are the weakest possibly glue joints you can make. I doubt if that's news to a veteran wood worker like you! Those long frame sections will go through a lot of shrinking and warping around during their lifetimes, and it's pretty much a given that within a few years those weak glue butt joints will break.
For really big and especially hardwood mouldings, "Tite Joint" fasteners will hold your frame together long after the glue is history. Counter top suppliers have them, there are several different types. You can build a router table jig to clamp the frame from the top registered to the miter cut, then plunge the router up from underneath to dig the big hole, then a secondary small cut for the cross channel. Not so hard with the right jigs. There's enough wiggle there so you can manually line up the pieces as the glue is setting, then torque it down. Not sure the biscuits are a good idea, too hard to register in the first place. Also a great way install frames too big to move into an installation space assembled, you also would stretch the canvas on site.
http://www.bizier.com/e/html/wood_countertops.html down towards the bottom of the page
http://www.louisandcompany.com/storefront/fasteners-assembly-fittings/other-fasteners/tite-joint-fasteners/prodKV516...100.htmlAnother option is a tumbnailer, worrisome though the name is.
http://www.youtube.com/watch?v=dmtRAB3S_SgThat's the hardwood solution for a lot of framers. I'm not thrilled with the plastic inserts, and would not want to do that on a huge frame.
I do a lot of polystyrene frames on the truly huge sizes, say starting with about 7 feet wide. They weight almost nothing, even with my bad back I can carry a 51 x 104" framed piece by myself, and I often do. Most poly mouldings look like crap, or at least like refugees from the Victorian Era, but there are a few that are positively handsome on the wall and I have never had a complaint. For poly, your strongest joint is to use Loctite "Ultra Control Gel" from Lowes or Home Depot, expensive though it is. Exceptionally strong joints. Poly takes a lot of practice. Use TCG blades to cut, use very fast (even karate-like!) chops, and you must exercise superb control of the glue which can melt the surface of the moulding in seconds if you get sloppy. Hint...don't get that stuff on your fingers! Do NOT underpin poly moulding, that will create tiny fractures that will greatly weaken the otherwise very strong glue-only joint! The trick is make perfect miter cuts to start with, which means a very well calibrated miter cutting system, a sharp blade, and a few tricks.
Edit...and there is a lot of skill and tricks involved with joining those poly frames by holding the pieces together with your hands for between 45 and 90 seconds. Just pressing them together flat on the table is not good enough, you need to be 100% sure you've got almost 100% contact of both surfaces. I got pretty good at that after the first 1000 frames.
Probably a good idea to bring this up on thegrumble.com where you will get a lot of good current opinions. That's where I learned about the Tite Joint things long ago, and was that ever a help.
FWIW, I believe the strongest v-nailers are named something like "Euro-nailers." They'll know of the thegrumble.
