I believe you can install the QMaster module on other computers with just one FCP license. You just install that module and no other parts of the Suite.
If you're using Compressor I don't think it matters what application is exporting, it's just Compressor pulling the frames from 1 running copy of Motion into the QmAster cluster and spitting out the final product. You may get better results though by just dropping the Motion project right into Compressor and exporting that; it will treat it like any QT file and you won't have to have Motion running during the export.
I'd be interested to hear your results with the Minis; and ideal cluster would be a bunch of towers but can a few lower-end machines do the trick as well?
JK
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SCQT! Self-contained QuickTime ? pass it on!