Brand name & model # of the A/B switch?  And/or a photo of the interior?  If it contains a physical 8-Pole Double-Throw switch, it might work.  But if inside is a 100BaseT network multi-port router on a chip, with a single pin to that switch, controlling the output of all the signals, it definitely won't work. Personally, I would bet against it connecting all 8 pins directly to all 8 other.  But a customer photo inside the (more rugged-looking, $17 instead of $6) CablesOnline.com model # SB-034 physical switch looks OK: Login to view image In theory the box will subject the audio signal to no shielding by ground thru routing the wires, but for short runs and where only audio signals are inside a metal box, you should be OK.  The only thing you need to make sure is that your box, like the one shown above, actually switches all the wires and doesn't take any shortcuts like connecting a bunch of them together, e.g. "because only 4 are used in 10BaseT".  Yes, in theory you only need 4 wires for Powerlink mk.1, but do you know which are which?  Don't chance it.  Also I don't like the potential shorting of sliders inside a multi-pole switch, so I would advise powering down both BS9000s (and thus the BL8000s are also told to power down) before switching and powering up only one of the BS9000.  In the unlikely event you get "the usual hum troubles and noise burst each time the volume button is pressed", switch to Mk.3 Powerlink cables from the box to your BS9000s.

P.S.  A manual switch for such sleek equipment seems rather declassé.  Given the one you proposed, I guess an automatic Almando Multiplay Stereo is way too expensive? Perhaps using one of the 9000s as the Aux-input to the other one might work...  On the other hand, who really needs a 12-CD changer nowadays?  By all means tell everyone what kind of cool installation you intend to do!  (Or as they say on the automotive forums, "If you don't post pix, it didn't happen.":-)

Although expensive I can recommend the Almando Multiplay for this usage. I am using an earlier version of this product with my Beoplay V1 and a Sonos Connect as sources. Have been using this for almost 10 years now without any issues. One of the connected sources are playing straight through. When the other powerlink input is activated it switches to that instead. As far as I know similar products called powerlink switch are offered by Bosscom and Oneremote. I have no knowledge of how well they work.