I think it will be difficult to make a web control panel both 03x and 04x compatible without some extended scripting. There are a number of major issues not the least of which is the fact that the entire 04x OS uses language conversion files for all printed text and therefor no script contains the actual text. This also aplies to the new 04x packages as well and there control panel addons. The main thing that makes it a bit more difficult at this point is that there aren't very many packages with any language files other than English and the main system currently only has an English translation. But if you really want to see what the language file does to the system you can play with the "censored.tgz" file and convert the LANGUAGE=english to LANGUAGE=censored in the ssyslinux.cfg file on a new floppy. Which I don't really recommend doing this to your main system, but it is fun to play with and can make your system much MUCH harder for anyone to mess with (even you).
So I guess what I am saying is that any control panel will work because it is just a web server and most of the files are in a very similar if not the same format. But the system it's self is VERY much changed as to how it works and the package format is a LOT different.
As for adding things to the web control panel or even changing it, all that is required is to place your files in the /boot/cpc directory. That directory is copied to /wwa/ at boot time. So even sub directories can be included under /boot/cpc/
and any file with the same name as the original files will overwrite the defaults.
Although be aware that those files end up being in the ramdisk. So size is of some interest unless they are just symlinks.
From a personal standpoint I would have to see the 03x version to really say how much work doing the conversion to the 04x packaging would be. But if you download the new "Pack" command there is a lot of information included in the base scripts. <a href='http://lewys-spot.dyndns.org/packages/0.4.x/non-package/' target='_blank'>http://lewys-spot.dyndns.org/packages/0.4.x/non-package/</a>
Dingetje also posted a small howto that I wrote to help a bit with the new package system, although I suspect it needs to be several more pages.
<a href='http://dingetje.homeip.net/dokuwiki/freesco/howtos/repack03xto04x' target='_blank'>http://dingetje.homeip.net/dokuwiki/freesc.../repack03xto04x</a>
If you are afraid that you might make a mistake. The chances are high that you will never learn anything.