Is it possible for non-mods to initiate tag synonym requests?
You should be able to - go to the page for the synonym you want to be the master (I think), click "synonyms" and there should be a "suggest a synonym" box at the bottom.
Users with more than 1250 reputation and a total answer score of 5 or more on the tag, can suggest tag synonyms. Users with a total answer score (total upvotes minus total downvotes) of 5 or more on the tag, can vote for tag synonyms. Suggestions will be automatically approved when they reach a score of 4, and automatically deleted when they reach a score of -2.
Click on any tag (e.g. cylinder-head)
For the cylinder-head tag, the resultant URL looks like:
Click on the synonyms link
Found in the blurb at the top of the page body.
For cylinder-head tag, the resultant URL looks like:
Suggest a synonym
Found towards the bottom of the page, with a "Suggest" button next to it.