I have two 486-ish bridgeboards - they're not that rare really. Well, actually, I have three, but one doesn't work, and that is rare, which is really annoying (it's an A2386SX modified to have a 486SLC33 onboard instead of a 80386SX).
If you're going to use a bridgeboard in an A2000, you may need to look at soldering the 16-bit parts of the ISA slots in. It's easy, but time-consuming (I did it on one of mine). If you don't, you'll be limited to an 8-bit soundcard like a Soundblaster II, and an 8-bit anything-else card (ethernet, IDE etc.).
I have an A2386SX which is upgraded with a 486SLC2-50 (8MB RAM onboard), Trident 256K SVGA card, Soundblaster AWE 64 and a no-name IDE/Floppy card, with a small hard disk attached. That's my Amiga in the living room, it's in a standard A2000 KS1.3 with a GVP SCSI card.
In my other room, I have another A2000, this time with OS3.9, an Apollo 2030@50MHz, 32MB RAM, Picasso II, Ariadne card, Catweasel MkII... and also a GoldenGate 486SLC which is powered by a Cyrix 80486SLC @ 25MHz. This has 8MB onboard at the moment. Graphics card is a 1MB Thunderbolt ISA (Cirrus Logic CL5422 I think), Soundblaster II (8-bit), 3-Com 3c509 (8-bit).
The Goldengate is the better integrated of the two, it uses Amiga floppy drives and Amiga partitions much better. In fact mine uses a 4GB SSD off the Catweasel (which has a Buddha built in). It's also better integrated with serial ports, parallel ports, the mouse, etc., and has a Monitor Master automatic switcher.
The reason I've posted all that stuff, isn't to gloat. 
(well maybe a little 
). But if it's of any help I can tell you how certain pieces of software runs on them. The thing to remember is that the 486SLC chips are not true 486-class chips, they're turbo-386s. They have a 16-bit databus, not 32-bit. Also remember the graphics cards are running on an 8MHz (or 10MHz if you want on the GG) ISA bus. Not fast.
As for how rare true 486/586 bridgeboards are - they don't exist. The closest was an SBC released (possibly) by Blittersoft, I think, but they were just SBCs. Much less cool.
That's really an interesting post!
I am fond of PC-Task 4.4, which i now use on ly desktop 1200 with 1260@50 MHz. The emulation is quite good, with games like Conquest of the Longbow, Dune, Prince of Persia, Monkey Island, Wolfenstein, running perfect. Of course i am limited to BC bipper as there is no soundblaster emulation. Alone in the Dark though is terrificly slow and unplayable.
I also own all the Commodore bridgeboards, and will be using the 386sx whenever i revamp my A2000.
I would be very interested in seeing some benchmarks from your different setups. Maybe you could download the DOS tool called MIPS and give us a screenshot of the result, to compare with PCTask, so everyone knows what to expect. I will post screenshot of MIPS on my setup this evening. Please tell us also what you get with the games listed above, especially Alone in the Dark.
About the 386sx, i think there are enough 16 bits ISA slots in the A2000 to get a 16 bits VGA and a SB16. Running emulation from a hardfile is not bad with SFS, instead of a dedicated hard-drive.
