What do you mean by "land on"? Notes that start or end exactly on the first beat of the measure?
It should be fairly easy to write a script that looks at all MIDI notes in the item and selects them if the meet that criteria. I'm not at my Reaper PC today (or probably most of the weekend
), or I would try to whip it up for you.
With all of the MIDI scripts available in different ReaPack packages, I wouldn't be surprised to find that someone has already written something like this (or at least very close to it).
If you're feeling somewhat adventurous, you might want to browse through some of the MIDI scripts people have written for ReaPack. If you can find one that selects certain notes, you could try to modify it to select notes based on your criteria.