Im assuming your talking about Omega in the contexts of ACI Anchorage Calculations.
Theres no exception for hold downs, however there is one for wall sill plate anchorage.
But what I have seen does it to use the limiting load clause in ACI anchorage requirements for seismic rather than omega. Also some anchor manufactureres have proprietary anchors which have a tested allowable load, to me this is a justifiable means to not apply omega on your hold down anchors.
Ductilie crush of the sill plate at the hold down posts is one method I have heard tossed around to limit the anchorage load. This is also mentioned in the commentary of ACI Ch 17.2.3.4.3 (Option (c))