Hi Gareth
I just tested this on my Mackie Control and it is working fine here, I'll run through what I did:
Create an arrangement with 6 audio tracks. Add two Aux Sends, creating two Aux Channel Strips and giving a total of 8. Setting up 8 Channel Strips is purely for testing/convenience purposes, matching the number of channels on the Mackie control.
Open the mixer, ctrl click on each Aux and select "Create Select Arrange Track. Alternatively, select the Aux Channel Strip in the mixer and hit ctrl T.
The aux channels are now showing in the arrange window and should appear on the Mackie Control. You should see audio tracks 1 to 6 occupying channels 1 - 6 on the Mackie Control, Auxes 1 and 2 are Mackie Channels 7 and 8 respectively. Insert some effect into slot one of Aux Channel Strip one. Now press the "Plug-In" button on the Mackie Control. On Channel 7 you should see "ChEQ" under "Aux 1"- Press the V-Pot and the parameters of the Plug-in should appear distributed over the 8 channels of the LED display.
If you have more than one effect in an Aux CHannel Strip insert, pressing the Plug-In button a second time will show all the plugins on the Mackie Control LED. Press the V-Pot below whichever Plug-In you want to access and it's parameters will be displayed and can be adjusted as usual. I notice that this does not seem to work if the Aux CHannel Strips are not showing in the Arrange window as described above
FWIW, I actually never resort to this, I use the Mackie C4, which is brilliant for tweaking Plug-Ins. In fact, I find that using it and listening, as opposed to mousing and watching has a subtle but significant effect on how I perceive or hear what a Plug-In is doing to a sound. It's a very interesting experience ....
Just FTR, these are the Mackie Control Universal Models, the old, Midi equipped ones, not the newer USB ones, although that should not make a difference as far as their integration into logic is concerned.
Here are some relevant System Details:
Logic Pro 9.1.5
OSX 10.6.8
MCU, XT and C4, connected through a Unitor 8 Mk II
HTH, kind regards
Mark