Yes, all are hackable. Some models will be more difficult than others.
Version 1 is OK - just add a hard-drive.
Version 2 and above have a crippled BIOS ROM that will only boot IOpener's QNX.
Version 2, 3, and some 4's have Epoxy on the BIOS Chip. You *could* flash these without removing the Epoxy or BIOS Chip, by formatting a hard-drive with a hacked SanDisk image on f r e e d r i v e dot com. I've done it only because the thought of chipping away Epoxy was not appealing. If you're daring, you can scrape the epoxy off by picking at it with dental picks, while applying heat from a heat gun. A hair-dryer might work, but it will take a LOT longer if it works at all. A Heat Gun and dental picks will take you 10 to 30 minutes. Be sure to pick the epoxy off between each "leg" of the chip -- amateurs often miss this VERY CRUCIAL area, thinking that only the top layer of epoxy is all that there is to remove.
Once the epoxy is removed, you can drop in a BadFlash ROM. If the BadFlash ROM won't boot, and you're certain you installed the chip correctly, you'll have to scrape off more epoxy and/or wiggle the chip upwards to make better contact with the socket. (Pushing the chip all the way into the bottom of the socket almost always results in poor contact)
Version 4 and 5 have dual-voltage support whereas Version 1-3 require a Resistor modification to support dual-voltage CPUs. You can identify a V4 or V5 by the Rise CPU, and a V1 through V3 by the WinChip CPU.
Version 5's use a new Sound Chip (that's how you identify them), and a new BIOS -- but at least they don't have epoxy. They all have the sought-after "SW4" cpu-voltage switch, which makes it VERY easy to drop in an AMD K6 chip. Some late-model V4s also have SW4, but don't hold your breath -- having SW4 and the older-style Yamaha sound chip is a RARE and very sought-after combination.
Version 5 BIOS won't let you get into the Setup Screen, in addition to the crippling introduced at V2. You'll definitely need a BadFlash ROM or other externally-produced ROM, since there is no way to get into the machine enough to flash the BIOS.
The hard-core hack is to add an AMD K6 chip, clocked at 300MHz or above. For these hackers, the V5 sucks -- you cannot tweak the core voltage beyond what SW4 offers. The hard-core way is to tweak the voltage using a trimmer pot. The hard-core hack needs additional cooling mods, which I won't get into here.