Above are five examples of well known marks on Mexican pieces, none of which bear a Mexico, Taxco or DF stamp. To gather them together, I went through about 80 or 90 item photos on my harddrive.
Granted, 5 out of 90 is a small percentage, but it indicates that Hougart's alleged "virtually impossible" is, at best, an overstatement and your contention that, "Mexican jewelry is always marked Mexico in some way, even the early stuff.", even farther off the mark.
One important guideline I've learned in my years of running this website is that in the world of silver marks, absolutes like "always" and "never" just don't seem to work and the easiest way to find an exception to any rule is simply to state that none exist. Of course, I learned this the hard way :)