While this doesn't exactly answer the question, when you're in the agenda tag view you can filter the tags by using
\ TAB and specifying another, this is probably the easiest approach.
The default method in org-mode is to sort the tag-match agenda view by the sequence they are seen in the files (10.4.3 of the manual doesn't say it's alphabetical but it could be wrong).
You can adjust how they are sorted by changing
org-agenda-sorting-strategy, so you might want to use
M-x customize-variable org-agenda-sorting-strategy from there you will be able to toggle all sorts of things.