Before this "get in on world rankings" business, my memory is that USATF sometimes has - and sometimes has not - allowed people to chase the standard after the trials.
Rules lawyers will want to look at
https://www.flipsnack.com/USATF/2020-olympic-games-selection-procedures-track-field/full-view.html, which is linked from
https://www.usatf.org/events/2021/2020-u-s-olympic-team-trials-track-field.
"The top three (3) place finishers in each event at the 2020 U.S. Olympic Trials – Men’s and Women’s Track and Field, June 18-27, 2021 (2020 U.S. Olympic Trials), provided they have met the 2020 Olympic Games qualifying standard during the prescribed period, will self-select
themselves via head-to-head competition for a position on the 2020 U.S. Olympic Team for the 2020 Olympic Games."
says 'meet the standard during the prescribed period'.
They define meeting the standard to include rankings, just above, and that's also where they talk about the 'prescribed period':
"Qualifying Standards for Men’s and Women’s Track and Fieldevents at the 2020 Olympic Games
Note: Athletes may meet the qualifying standards in one of two ways: The qualifying standards as established by World Athletics, which must be achieved during the period May 1, 2019 through April 5, 2020 and from December 1, 2020 to June 29, 2021"
So, the 29th is either the last day to hit the standard, or the first day that is too late to hit the standard. They say "through April 5" in the 2020 language but "to June 29" in the 2021 language. Since the 1500 final is on the 27th, though, I think this means hitting the standard on the 28th after taking top 3 in the final is definitely allowed, and doing the same thing on the 29th might be.
This is of course assuming USOC and/or USATF don't just decide to do whatever they want and ignore the stated procedure - somehow I have the impression that's happened, at least for entry into the Trials... I'd guess they are able to do that as long as they put forth athletes qualified according to the IOC.