Wonderful! Seems obvious that I should have been looking in Form edition now that you said so… 
Just for the record, default="today" (and variations as suggested by @markp ) also works perfectly with PartialDateUI
<cell type="field" id="5" uitype="plugin" name="this" initialize="name=PartialDateUI;df=catalogedDate;tp=catalogedDatePrecision" default="today" uifieldformatter="Date" colspan="3"/>
Thanks! 