There are a lot of projects that might not merge a PR within a month and the further you are into October the more unlikely it gets that the PR is merged in time. As such I do like it that my contributions to a project do count, even if the maintainer isn't around right now...
I did think about it when writing the idea, and in the end it still looks to me that it would be worth it.
This rule would mean contributors would actually care to initiate a conversation about their intented work, and would probably drive them towards projects where there is some activity and positive feedback from the maintainer, instead of projects which didn't want or ask for any contribution, like it happens now.
Also, I think such constructive environment could drive a reasonable maintainer to help the contributors by fast-tracking the PR in case the end of October was approaching. And by taking into account the PR merge date, not the PR creation date, it would mean you as participant could start working on some PR even before October.
Because that would mean a very small amount of t-shirts go out. I think people think of DO a little too much. This isn't some altruistic action to better Open Source. It's a marketing gimmick.
Less shirts going out == less shirts being worn == less DO's name plastered on every programmers' bodies.
This is not to say that Hacktoberfest hasn't been helpful for some Open Source projects but let's not pretend that DO's main goal isn't marketing.
(BTW, there is nothing wrong with that, they are a company who wants to earn money, and this way, they help Open Source a little bit too)
They shoud worry about the bad pr they get from the consequences of their careless marketing actions. But granted, we won't collectively start making t-shirts about it, so I guess in November everybody will forget and move on.
A D.O. representative talked about similar concerns on Twitter... my thought is that there are enough people able to do a plain and simple PR for giving out 70k t-shirts. This is nothing more that a corporate pr stunt, not a long-term opensource awareness effort, so it would be ok to just give out the t-shirts to the first valid merged PRs and call it a day.
Regardless, IMHO if someone wants to do a PR on your repo, asking you should be their first step.