Is This A Bad Move?

If the speakers really are an exact fit then it won't matter. If there's a space around them then of course some sound from the back is bound to come out around them in front. The high tech solution is Owens Corning 703 acoustic panel, cut to fit behind and/or around. But you could just as well stuff a blanket or pillow in there.

Make sure you fill those holes with silicone

mate, i had a pair of Advent legacy II and they were shite and I don't rightly know why anyone ever thought those speakers were worth more than their weight in feathers... but if ya gotta do it, give it a try.... maybe you get some low end re-inforzament from the woofs

Thanks for the constructive response millercarbon….the bookcases have a framing/molding that is exactly 26" wide, then the actual shelf area opens up to about 28" wide. Both sets of speakers are a shade under 26" wide, which will leave about an inch of space around the ends. There will be about 3" of space above the speaker tops, so I can fabricate a piece of material to fit those areas to keep reverberations down to a minimum. Even though this is my 'vintage' system (one of two, actually), I want to do whatever I can to make it sound its best. Your suggestion seems like a good one.

Millercarbon is spot on with his comments regarding your bookshelf.  Interestingly,  Michael Kelly (formerly a/d/s and then Aerial) incorporated Environment and Treble controls on his large Aerial LR5 monitors for the same purpose you are discussing - different set-up situations.  The settings are Stand/Shelf/In-Wall.  I am not sure exactly the effect of the controls (mine are out in the open on stands so I use the "Stand" setting) but it might be worth looking into whether you could use EQ to achieve the sound you want from the bookshelf location.  Most computer sources can accommodate some sort of EQ programming.

I owned the L810s and they sounded great for their time and still pretty good today. Be sure to fuse the tweeters appropriately since it was not unusual to blow them.  I wish I had never sold mine.

disknic, the problem is not so much how the speaker sounds, it is how the cabinet sounds which is dependent on its construction. 1/4" plywood backs are going to resonate like a drum head and other loose parts are going to vibrate and potentially make a lot of noise. 
The best cabinets for bookshelf speakers have no backs at least in the bookshelf section and are solidly constructed with fixed and dovetailed shelves at least an inch thick. Knock down construction is a nightmare.
If you wind up with a few buzzes you can find them by damping various parts with your hand. Once you find them just jamb a little butyl glazing tape in there and that should stop it. You can get the tape at your local Portland Glass dealer or any place that glazes windows.
The large Advents sound best upright on stands. Why are you putting two pairs of speakers in the cabinet? Doing that will ruin your imaging if you care about that.

Thanks for the suggestion, mijostyn….I'm using the bookshelves because I just picked up the Advents a month ago or so and don't want to ditch the ADS L-810s. I won't play them together, just one pair at a time. The units are handmade, not knockdown, and I won't be playing music at a loud enough volume to excite any buzzes, hopefully. The shelves are secured really well to the cabinet walls with metal hardware, which should help. They were made by Thomasville Furniture here in NC, which is no longer in business (along with many other furniture companies). The room doubles as a den/office, so space is at a premium.

If you can get just one sheet, OC703 one inch thick, cuts with a knife and you can fit beautifully around and behind. This will kill all sound and most vibration. Its a yucky yellow fiberglass but wrap it with the right fabric and it will look pretty darn custom professional. Wouldn't need much and the whole thing would be crazy cheap yet effective.

I'll definitely give the OC703 acoustic paneling a shot.