No engineer here so this is just IMO
I don't recall that the TF standards under development mention this but did remember it in the recently published ICC log standard. So here's their poop
302.2.4.3 Kerfing:
where kerfing is provided in logs used in walls the depth of the kerf shall be no deeper than H/2. The sum of the depths of the kerf and cope shall not axceed H/2. Where beams are kerfed, the net section shall be used to determine the section capacity.
I think the last sentence says what you need. I don't think 1/2" is going to accomplish much in a rectangular timber. In log work 1/4-1/3 depth is typical. You are encouraging a check, a failure, and directing it in the case of a beam to a better, safer,face. No need to be shy about it.
