Hi Joshua,
If you use a nested default program in the engagement program, you can add additional flow steps to the email-sending smart campaign that's referenced in the engagement program stream to control the send time a bit as a workaround to Recipient Time Zone.

I just have a couple examples here - you'd want to account for all time zones in some fashion but you can use "starts with" to group all Europe time zones in one step, for example, if approximate send time is good enough. It's unlikely that it'll be worth the time to set up a flow step for every individual time zone. Use wait steps between to account for the time differences.
You'd also want to have one final flow step to send to anyone who was not accounted for previously, such as those who had a time zone change while the campaign was running:

I don't believe, however, that this method works with Champion/Challenger tests - based on this community thread you may have to also incorporate random sample in order to send different versions of email content. That could get quite complex. Perhaps you can consider what is the most critical component you want to account for - the send time based on time zone or the tests.
Cheers,
Beth Massura
2022 Adobe Marketo Engage Champion