This should allow you to define an MT app to use (say) only 4 of 8 threads. That would leave the balance for GPUs or other CPU ST tasks.

From: David Anderson
Subject: Re: [boinc_alpha] app_config.xml Request - Optional <plan_class> element?
Date: September 6, 2013 3:44:09 PM PDT
To: boinc_alpha

I implemented this; app_config.xml can now contain <app_version>
elements that override the parameters of specific app versions,
including #CPUs, #GPUs, and cmdline; see
http://boinc.berkeley.edu/wiki/Clien..._configuration
This will appear in a future release.
-- David