Is there better way to 'touch' a row than performing a dummy update?
Well, if you want to change the xmin, then it needs to be ensured that other transactions can still see the old version of the row, so you need to make a copy with the new xmin, which you do with an UPDATE.  You can reduce the impact of this by updating a non-indexed column, to get the benefit of HOT.

Of course, this whole line of thinking is dubious, and perhaps you should be using some other facility altogether.