I don't see anywhere where you can add custom codes in HOBD.
To answer your other questions,
1) I wish we could replace iGo8 with Primo, but unfortunately we can't just yet. I tried replacing the stock ezrider.exe with a Primo exe renamed ezrider.exe and Primo does boot but the "System startup, please wait" message does not go away and the unit reboots in about 5 minutes. The system seems to be looking for some confirmation from ezrider that iGo8 booted OK and Primo does not provide that confirmation. I searched and searched but couldn't find anything on how to remove that message, You can set Primo to autostart, it is a minor inconvenience waiting a little longer but at least the unit boots to Primo.
2) I think Primo was designed for dash-top units with batteries and it seems to get confused and display that message. There are many sys.txt commands and their explanations on many sites, maybe all it would take is the right one to insure that message does not appear. Again, it is a minor inconvenience that wouldn't be worth the time it would take to find a fix.
3) Maybe, try the auto launch option. The AV windows seem to go overtop of the window to the program they see upon launch (Igo8). Note if it did display that information it would overlap the bottom of the screen where Primo has it's info bar.
4) That would depend on the skin and their aren't too many fro Primo 1.0, especially for 800x480 devices. When I started using Primo it was on my WiMo phone with it's tiny screen, and large signboards were required to view them properly. Perhaps that is the reason they are so large, because it was made for devices with smaller screens? Funny, I remember preferring to use my Phone for navigation over my AVIC just because I found Primo so user friendly. Anyways, look around and you may find a skin for Primo 1.2 that allows for smaller signboards.
On another note I see that there are versions of Primo 2.0 available here and there, with licensing issues unfortunately. I can't wait for a cracked version with 800x480 support 