What about an across-the-board A+ standard for all events? In 1972 Lee Evans didn't get to run in Munich because he was 4th in the OT. (He was on the relay but it ran out of runners when Wayne Collett and Vince Matthews were thrown out and John Smith pulled a hamstring, leaving the relay with only 3 guys. Matthews, btw, wrote a fine book, "My Race Be Won", which is out-of-print.)
I don't know the standard for the various distances and field events, but something equivalent to, say, 2:06:00 for the men's marathon. They wouldn't exhaust a country's spots. You could have 3 "A"s OR 2 "A"s and as many "A+"s as you have.
Athletics Kenya and the Ethiopian federation would resist losing their control over athletes (USATF's control is limited by the top-3 rule or I wouldn't trust it to do anything to lessen such grips on athletes as it has). In the end, though, the prospect of a marathon-field laden with Kenyans and Ethiopians would be worth it.