Thank you all for your helpful answers!
I've been meaning to post a reply for a while, but time flies as I'm sure you all know, and I seem to have some problems striking a balance between spending too much time in these forums and waiting far too long before getting around to answering ;)
I have managed to assign a keyboard shortcut to the bookmarklet in Fluid. It does indeed seem like assigning keyboard shortcuts generally just doesn't work with Firefox. Pity. Or maybe just the push I need to switch to Safari…
However, the bookmarklet still doesn't work well without a mouse. When using it, the quick entry window pops up, but it isn't active, so I have to either click it with the mouse or cmd-tab to OF to be able to change the text or just save it.
I'm wondering if this could be related to
the other problems with the QE window in OF 1.7, but it seems to be a different issue. Maybe the bookmarklet script can be tweaked to fix it?
Just using the global clippings shortcut, as whpalmer4 suggests, is an option in Safari (it doesn't work in Firefox). The problem with that method is that it only works if you mark a bit of text in the email -- at least on my Mac.