This is also apparent in FTD v6.3.0.1.
No functionality within the FMC GUI (v6.3.0.2) to carry this out.
If you attempt to deploy configuration using FlexConfig, you can create a class and assign it to a policy-map.
If you then attempt to use the ASA command 'priority' under this, the FMC will claim successful deployment of the code to the FTD appliance, but by running "show running-configuration" you won't see this entry within the ASA code.
Also trying to enable a low latency queue on an interface using 'priority-queue <interface>' won't deploy to an FTD device - FMC will claim an unsuccessful deployment of the config.
TAC have confirmed that this isn't yet a feature, although this enhancement request doesn't indicate a plan to implement it in an upcoming release.
The workaround suggested was to enable fastpath for any traffic requiring priority, but I don't see bypassing IPS features as a suitable solution for lack of this functionality.