Maybe this in reallity is two projects:
1) A very simple one taking 7+ V from a robust source.
2) A somewhat more complicated (and expensiver) one taking undervoltage and nursing delicate rechargeable batteries.
I think it's actually three projects
1) a buck converter for the high voltages. Say starting at 7-8V and going up however high the components you chose can tolerate. The LM2576-5 suggested by meltwater seems good here at least for a hobbyist soloution.
2) a low dropout regulator to cover the 5.5 to 7V or so (more with a heatsink) range cheaply and simply. Something like the L4940V5
3) a boost converter for running from voltages below 5V.
I don't think protecting the batteries is too big a deal, afaict most lithium ion packs have overcurrent and overdischarge protection built in anyway.
You COULD build a sepic or flyback converter that could do it all but i'm not convinced the complexity is worth it.
I guess something using LM2576-5 would work then for the 7+V range. http://www.ti.com/lit/ds/symli.....lm2576.pdf
Looks a pretty good choice for the hobbyist. The low switching frequency should make it pretty tolerant of poor board layout. Downside is that low switching frequency will mean a relatively large inductor.
However, would a Zener diode regulator work, or would the current draw be way too much?
It'll probablly work if you can find a beefy enough zener and you can tolerate the high input current but it's a pretty horrible soloution.