Easy. I use Openbox as window manager. And lxpanel as panel. So yeah the panel is from a DE already.

Then.. I use K3b for CD burning, Konqueror as a light browser, Dolphin as file manager, Ktorrent as torrent client, Ark as archiver, Gwenview as picture viewer, Kwrite as text editor. Thats for KDE (4.2.4).

Then I use pidgin for messaging (that can be gnome,xfce,whatever, gtk), claws-mail for email (gtk), monodevelop as IDE (gnome), Gedit sometimes for text editing (Gnome), Xchat (gtk), Gimp (GTK)..

You know.. I don't really care about such stuff. I always try to use KDE as base , and some GTK stuff as support apps, and if I REALLY need..then I install a gnome app with a hell of dependencies (yeah I hate gnome, so i cant really help it.. I struggled with it for months when KDE 4.0.0 came out).

So what I want to say? Even with this much "cross-app", I only get ~200mb or ~300/400mb memory usage. What is... OK I think. Even if I got 2 "workspace" full... :)

Since every distro is somewhat different, you could only use a bash script (that supposedly runs on any distro), which determines the current distro name, version, and so on. Such things like "it runs on everything" is almost non-existent on linux, so you better not hope too much.

Its possible to write an application what runs on everything IF that install got the necessary libs. Like you write your app in python, everyone can run it IF they got the right version of python... Not easy.. :)

Install.. the universal method is the "./configure .... && make && make install" as you may know it, that *should* work everywhere. But you meant in packages , so yeah.. that is the first part of my post.

(There are many utilities for every distro to modify the default iso, and so on, so you can make your own flavor, thats not the problem.)
So in a nutshell:
(The script shall be written in bash, should detect the distro name, version, and be aware of those stuff, and then install the application in the correct way)

(Of course, all of this are my personal opinion, others should correct me, etc... anyway, good luck.)