'Kay, thanks. I've installed the v4 bootloader, so now I can answer the rest of my own questions:
- The bootloader version is displayed during startup; I just wasn't paying attention. It's the second line that appears on the screen during startup (just before the battery voltage).
- The change in "hold switch" behaviour seems to affect mainly the on/off switch rather than the hold (same switch but opposite direction on the X5): v2 boots rockbox as soon as you push the slider into the on/off position; v4 briefly wakes up but immediately shuts down again (from the bootloader, before loading RB) unless you hold the switch there for half a second or so.