You can actually tell ARK to overclock only in specific games, since the settings system uses the same exact format (and code) as the plugin system, you can manually open settings.txt file and do something like this:
ULES0000, overclock, on
You can't do this in the recovery menu to keep it simple and easy to use.
The easter egg is a message that appears when you install and run ARK in a very specific way that isn't meant to be.
The only tip I can give is that it's for Vita only.
Glad you're loving it!
You can install plugins like Custom Firmware Extender to bring back those "missing" features (like UMD USB mounting and setting a very specific clock), highly recommended as it looks really nice and modern so it blends perfectly with ARK.
If by "UMD ISO mode" you mean the ISO Driver (Inferno, M33, etc) then that feature can't be brought back to ARK since there's only one ISO driver available (Inferno 2), which is more than you'd ever need anyways since it has less bugs and more compatibility compared to previous drivers. If you were to replace Inferno 2 with another driver like the first Inferno or M33 then you'd loose a lot of features (like 8K blocks, JSO and DAX, etc) and some homebrew on PS Vita might stop working (including the Custom Launcher itself) unless you do a workaround like mounting a fake iso (which is what Adrenaline and ARK-2 do). All in all there's no real reason to mess with ISO drivers in ARK-4 since the one (and only one) packed in has all you will ever need.