Is there any way to force the z-order of markers in Google Static Maps?
There doesn't appear to be any way to specify a z-order and the z-order does not seem to be determined by the order of the markers in the static maps URL.
Currently this is not possible.
However, there is a feature request in the public issue tracker:
https://code.google.com/p/gmaps-api-issues/issues/detail?id=2096
Please star this feature request to express your interest and receive further updates.