If you launch gkrellm outside the slit (that is, as gkrellm & without the "-w"), you can make it show on all the desktops by making it sticky. With a normal window in fluxbox, there's a button on the top left corner you use to stick the window, but because gkrellm is borderless, I believe you need to Alt+RightClick on it (EDIT: sorry, I was thinking of a different wm. In Fluxbox you need to middle-click on the desktop to get the Workspace Menu, select the current workspace, select gkrellm, and then you can do the rest of this-->) and you will get a window menu for it, where you can set the level (I set it to the destop level so other windows can be moved over it) and the stickiness, among other things. That way gkrellm will be on all the desktops and you will still have the slit free for whatever else you need it for.
Here is a good bit of info about Engage:
http://get-e.org/User_Guide/English/_pages/4.2.html
Since you are using Engage outside of the new Enlightenment, you will want to read the parts about the standalone version. Transparency is very easily done in the standalone (the module version is for E17 only):
Code:
engage -b "#00000000" -B "#00000000"
There are a number of other command line options you can use to configure the look of Engage, and then of course you can throw the whole line into your ~/.fluxbox/startup file and it will be taken care of when you start!
You should check out this page as well:
http://get-e.org/User_Guide/English/_pages/3.3.html
Engage uses .eap files for it's icons, and you need to have them for whatever applications you use, otherwise you get green question marks in the task bar portion. If you look under the Themes section of Get-E.org, you will find a number of pre-made .eap sets. Being a regular user of E17 myself, I have a number of them created as well, which I could post if you'd like. (Oh yeah, and the standalone version of Engage can also make normal eap icons look like the Gentoo theme glass-globes too without making you edit anything.)