If I want to use AU plugins, I pipe the audio through soundflower into AU Lab, run it through the plugins, and back into Max. It works pretty well. Now that I think of it, using an AU host that will accept control data for the plugins will be even better--then you can send that data from Max also.
Ah, I neglegted one detail! In the Audio MIDI Setup, I created an aggregate device driver (in Audio MIDI Setup>File>Open Aggregate Device Editor). I combined the built-in audio with the 16 channel soundflower. The result is one driver with 18 inputs and 18 outputs. If I process stereo audio in max first, then I may use the first two soundflower channels to send the audio to AU Lab, then the next two soundflower channels to send it back to Max. Or, I can begin with AULab using the built-in audio input and use soundflower to send to Max, etc. If I connect another I/O device, then I can build a different aggregate to include it. This setup is great for Max, because it allows Max to use different physical devices together, seeing them as only one driver.
I have had some trouble in this. Sometimes when I launch AULab and choose my 16+2 driver, the I/O setup dialog leaves off two of the channels. A couple of times I have had to rebuild the aggregate driver. I don't know why, but it only takes a few seconds to do it, so it's not a big deal at this point. I assume Apple will work out that bug in future versions.
with bidule by www.plogue.com you can load a au into a vst host. you can also control plugin parameters. its cheaper than kore by native instruments.
if you're working on a intel mac you need to be careful: a host compiled for intel (like max universal binary) can only host plugins also compiled for intel. rosetta - apples emulator for powerpc code - can only emulate ppc plugins if the host is also emulated i.e. (only) compiled for ppc.
ps: never tried bidule on a intel mac thought. but it's a universal binary.
I think the aggregate device driver was new with 10.3 or 10.4. As for MIDI mapping in AULab, I know you can map to the application itself (command key is the default), but I haven't tried with plugins. Too bad if it doesn't work with plugins. Try it with an instrument plugin assigning a Max midi channel for the midi input...
Okay, I played with it some more. Incidentally, I am using a PPC (G4 PowerBook), so I don't know about any of this with Intel.
I found that MIDI control from Max to AU Lab effect plugins works if the plugin supports it, and different ones support it differently. If the plugin has a fourth menu (view), then look there to see if you can select Midi Effect Editor. That will let you choose a MIDI input and channel.
In Apple's implementation (AULooper) the editor gives a complete mapping grid. In Ohm Force Frohmage, you can turn on AutoMapping in the settings menu. In Musolumo, control click a control to turn on midi-learn. Many of the Destroy FX plugins have a midi-learn button on the interface. In Luxonix LFX-1310, control-clicking on a control gives a contextual menu from which you can select midi assign or midi learn. Most of the plugins from the smartelectronix group (besides destroy fx) don't incorporate midi control.
In summary, what I found is that midi control from max to AU plugins works in AU Lab so long as the plugin supports it.