I received my ACA500 today. I'm still waiting on the CF cards so I'm only testing it with floppies for now.
The good news-- it does work with my A500!
The bad news-- I didn't realize I would be losing my trapdoor's RTC. In all my research before purchasing, this point was either not discussed or not a big deal to those reviewing it. To be honest, I will probably be using the system mostly for WHDLoad and the need for accurate time won't matter much there, but I would like it for working in Workbench 3.x or above.
There is a blurb in the "short manual" regarding A1200 accelerators. It says "If the A1200 accelerator has a Realtime-clock, it will not be used by the A500. Instead, only the RTC that is located on the trapdoor memory expansion or the mainboard of an A500+ will be used."
I'm confused because my RTC *is* located in the trapdoor and yet it does not show up when I booted into WB 1.3. Or, is this trying to say I need an A500+ for this?
I was concerned about the available RAM as well under WB 1.3. I went through the various boot options but wasn't seeing all the RAM I was expecting. Unfortunately, I do not have a floppy with Nic Wilson's SysInfo handy.

My A500 has the 1MB CHIP hack (512k built-in plus the other 512k from the trapdoor expansion). I only saw a maximum of 2.4MB showing up on WB 1.3. I was under the impression I would see the ACA500's 2MB as FAST RAM and the built-in 1MB as CHIP. It has been a while since I used a real A500 so perhaps my expectations are fuzzy.
When I chose the Kickstart 3.1 boot option (WB 3.0 floppy), I see .9MB CHIP RAM and 1.4MB FAST. I do understand that part of my FAST RAM was gobbled-up for the KS ROM mapping in this scenario. Since my A500 has Kickstart 1.3 in ROM, shouldn't I be seeing all of the available memory under WB 1.3?
Cannot wait to get the CF cards. The short manual also recommends using KS and WB 3.1 for the highest compatibility with WHDLoad. Really? I still have some other, older WB 1.3 compatible software I wanted to try out in a real WB 1.3 environment. It was my intention to make the first partition for WB 1.3 and work with that initially. I figured WHDLoad was fully backwards compatible with WB 1.3 as well. That's okay. As long as I can make another partition for OS 3.x and WHDLoad, that will be fine.