I did study this a bit,sorry this might be a bit excessive and poorly written.
As is already known the 500 is using the Renesas SH7727 cpu (Renesas is former known as Hitachi), the service manual is available online and can easily be found and tell you all the info you need.
And that is all we know about the whole subject.
The developement tools are also readily available for free by Renesas website only by registering an account. The only tool missing would be the E10-A usb which is the original debugging hardware interface (known as emulator), this would cost alot of money to obtain. A good start would probably be the manual of the processor (around 1000 pages) which would get you more familiar with the sh3 assembly.
Some more things worth investigation would be the e4m port (the one on the battery compartment) plus the debug port (mentioned on the manual and can be found after full dissasembly). For the first port called e4m I have no idea what it is but for the second port I think it's a bit more interesting and ivestigation would be interesting. It is a port called h-udi by renesas and it is a propriatory debugging interface connecting to the E10-A emulator, But it is actually a jtag debugging port, I don't know if the protocol is the same but i've seen debuggers online claiming h-udi compatibility via jtag. Essentially you should really be able to connect through jtag and have most of the jtag operations available.
That's as much as I personally know about the hardware, more on the firmware now.
I saw a post about reading the operating system from the memory . This is too much hassle for no reason I think since the firmware is already available through the update.bin provided by akai.
You can check the firmware through any hex editor or by using ida-pro for dissasembling. At this point I have to mention that changing strings on the firmware is really easy and even bypassing the checksum for flashing it again but this is as far as it goes. I know it's nice to discover some info inside the firmware but these are not code but data. I forgot to mention another basic tool, high-performance-embedded workshop (obtained also by renesas), this is the ide and is really useful, you can even emulate and debug on it.
I think this is almost all the tools and all the available info. Decompilining, dissasembling the firmware would be although really difficult. There is no dissasembler nor decompiler of the firmware that will provide you with usable code, it will provide you chunks of code you can understand and then rewrite. So you essentially rewriting the whole firmware by looking at what it is doing. Probably impossible, but this is a big word. Another case would be to write small patches to patch the firmware on the fly without using the actual firmware code (an example would be the magic lantern for canon cameras). Actually it would be easy to write some piece of code on the ide and run it on the akai (and seems much more doable), a hello world or something, but this is probably as far as it gets if you change the whole firmware (an example of this would be the work done one homepatrol by john doe on a simillar sh3 renesas cpu) .
Now for some candy
I did extract the strings from the firmware and there are some interesting mentions. Multiple mentions of hard drive would probably indicate hdd expandability on the 500. Also there are the strings for a test menu where you can test every single aspect of the 500 and also you can test audio-through, which you probably already know what it means. (I haven't seen anyone on the internet knowing how to access the test menu. )
As it seems, hardwere-wise the mpc1000 and 500 are really simillar, the cpu though is the same(correct me if I'm wrong here).
So if you had the source code for the 500 then you would probably be able to run the 1000 software on it, plus modify it with a 1000 screen and you have the ultimate machine.
Ok let's stop dreaming.
This is as much as I know, I hope someone catches on this project. I know I won't. If someone does though I think these are more than enough to get you started.