Use onscreen keyboard? Configure it to work with shift + hardware key? Another alternative? Or is it horrible that there's no tab key?

For the hardware keyboard, the tab key can be accessed by pressing the Chr key and then selecting the tab key from the bottom of the screen. Yes, it's two taps, but no, it's not _too_ bad.

As far as for the usage, for most use cases the cursor/rocker key on the left side of the keyboard does really the same thing. Press cursor down to move to the next text field. (I'm biased on this issue, but) there's really not much need for the tab key, except in xterm use - but there there's ctrl-i for tab, iirc.

Maybe you can change it with xmodmap

And here I planned to get emacs running!
If you ever write code, or latex, or any kind of semi-formatted text, then you'll hit tab all the time. It's almost as much used as the return key, particularly if you use emacs (because the tab key tells emacs to intelligently indent the line, and other things).