Cloning is certainly possible, but it might not save you as much work as you hope. Apart from anything else, it works most reliably when you start with a stripped-down rootfs and then install your apps in the clone. You say that you want to avoid that.

It sounds as though you are irked by the downloading more than the installation, and particularly by the fact that the files you need are not always available for download when you want them. I find that a pain too. You can avoid the repeated downloads by downloading and saving .deb and .tar files through a browser instead of installing them from the repositories through App Manager. Obviously this means you have to get a clean download at some time, but then you don't have to do it again. You can store them on a PC hard drive, if you don't want them taking up space on your 770. (In fact, you can do the download to your PC, probably more conveniently than to the 770.)

When it is time to install, you can copy the .deb to the media card and use the "Install from file" option in App Manager. (That is simpler than using dpkg.) It sounds as if you have used Linux, and so you probably already know what to do with .tar files.

If this is more installation work than you want to be bothered with, there are other approaches, but they are more risky. I use a backup as described in http://talk.maemo.org/showthread.php?p=33348#post33348, but my purpose is different from yours and my backed-up image is a stripped-down one.

The closest I can suggest to what you want is the following, and you will need a Linux PC for it.:

1. Make a jffs2 image of your rootfs, using the three commands in the first box of the link above.

2. Tar your rootfs.jffs2 and save the .tar to your PC.

3. Download the 0xFFFF flasher from http://www.nopcode.org/0xFFFF/ and install it in your PC.

4. When it is time to reinstall the rootfs in your 770, untar the saved rootfs.jffs2 and flash it to your 770. [Edit: Removed a misguided instruction to untar in the same directory as 0xFFFF.]

WARNINGS:
(1) I HAVE NOT DONE THIS MYSELF; [EDIT August 2010: I have done it several times and it works perfectly.]
(2) 0xFFFF is experimental, and the nopcode website looks as though nobody has done any work on it in the last 12 months;[EDIT: No worries, it still works from command line, though the graphic version is on hold.]
(3) obtain md5sum for rootfs.jffs2 before tarring, and compare it with the md5sum for the image you are about to flash.