RFC 822 expects date fields to be separated by spaces. However, I would recommend a limited set of options rather than the fully formattable sscanf codes that Ari first mentioned. If not, then the format can be specified by simple boolean options. However, I'll bet people will want it configurable.
Current practice uses a space, but perhaps a comma is better.
My primary concern about this is the extra work it will require of the server authors. Provided that the fields are well defined and can be parsed unambiguously, there should be no problem for log analyzers. I think that decision should be left up to the server authors.