JustSomeGermanDude
Experted expert in expertness.
You can put ads on your trailer video, but I think they will only show if they watch your trailer on the video page. If they watch the trailer on your channel page (where it shows the unsubscriber trailer), ads do not show up on it whether it is monetized or not.
Nope, not true, seen differently.
So:
I say no. The trailer is an ad for my channel. Why put an ad on an ad?
DON'T put prerolls or overlays on your Trailers! That trailer isn't there to make you money through views on it but through gaining you subs. Streamline it for that purpose and don't put distractions and turn offs on it.