At this time there is no way to change the default location for where the MCU joins computer objects.
When joining a computer object via the MCU the option is available to change the path to where the computer object is joined to after clicking the join option.
Additionally, when joining via command line, the OU/Container can be specified as well.
Please see the following for information regarding command line join location changes:How do I join to and put the computer object in an OU? Specify the join location. Join to a particular OU.