Add a second field tag (placeholder / runtime argument) similar to <<>>, but it would user a logical check for no value and would either omit the argument (harder would require something like a regex removal) or put in the "%" for the end user. I was thinking something like <{}>, <[]>, <()>.
Something you can do as a work around: WHERE xxx LIKE CONCAT('%',<<search term>>,'%') This way the users don't have to add the %.
We also have :all now, that works on drop down lists and adds an entry with % that is pre-selected. Having a % for date or free text is probably not as easy to resolve.