Why not? The 240 has higher resistance - may be it will need a little more heatsinks. But in general I think the 540 is the better device (looking to the specs).
Pay atention that there are PNP and NPN devices in the hybrid circuit.
A russian DIY-er reports thet there is DC at the output until it is warmed up.
If, perchance, the circuit you're referring to is the one at digilander.iol.it/essentialaudio, you might want to take a look at http://www.tubecad.com. The editor there runs a webzine, and the March issue has a lengthy discussion about a hybrid circuit that's sorta Zen-ish in the letters section. Even if it's a different circuit, you might get some interesting ideas from the discussion.
Re: IRF540 vs. IRF240. It should do just fine. Take a look at the SOA chart for the IRF540 at http://www.irf.com. There's a little more to it than just the voltage, current, and wattage ratings; it's the combination of them all, along with the package limits (in this case, a TO-220), duration of pulse (in your case infinite, i.e. DC), etc. etc. etc.
IRF240 is a TO204 device, large metal can with ca. 1 square inch heat contact area with heatsink (through isolating washer).
IRF540 is a 220 device with about 1/4" contact area....
When the device is dissipating 100W for arguments sake, you will typically have at least 30 deg higher substrate temperature with the smaller device. Of course you could mount it on a large piece of copper to simulate a larger area, but you will be adding capacitance as well.
So, even though they appear to be similar, you are going to loose a lot more heat performance through the thermal washer. You will need more heatsinking for 540 than for 240, probably much more unless you use many parallel devices.
Have you considered using IRFP240 instead? This is a 247 device. IRFP244 works as well, and they are not all that expensive.