Cynan - I like your line about using fix 'one-shot' and 'not that often, either' - I'm probably misreading it, but it gave me a chuckle!
What JGM said is spot-on for authentic tintypes - the silver enriches the developer, building-up the image by laying down a thicker coat of silver. I suspect though that by 'modern tintypes' Cynan does not mean the original, authentic process, but probably something along the lines of a gelatin emulsion like the Rockland kit uses rather than the traditional use of collodion.
I found a thread on a cinematography site in which one of the writers says he adds 'sodium thiocyanate' (extremely rapid acting fixer) to Kodak D19 developer for punchier images, the resulting brew being Kodak D94 for reversal processing.
There are also 'monobath' developers which contain a lot of fixer, but they provide ordinary negs - I think - not transparencies or 'punchier' negs.
Some developers (I think 777 is one) have to 'seeded' or 'ripened' before use to give their best. One way of doing this was to run any old bw film through it. Perhaps this added silver to the developer? If so, perhaps this is the idea with adding some exhausted, silver-rich fixer to the developer. Maybe any life left in the fix is rapidly neutralised and the silver is left to enrich the image?
After looking at the Rockland Colloid site, it appears that they use a reversal developer for their tintype kit, which uses metal sheets coated with AG+ emulsion. The MSDS sheets show that it contains ammonium thiocyanate. The site has a FAQ section where they caution against using ordinary paper developer. Someone on the PhotoNet site has posted about using the rockland kit calling it a "modern emulsion/reversal developer combo" adding that the surface of the finished tintype has a leathery feel, but more of a metal plate feel can be had by skipping the reversal dev step and using a dektol/exhausted fix combo, but this apparently reduces the nuances of hand-applied emulsion.
It's a very interesting thread - I hope someone more knowledgable about the matter adds to it!
Regards,
Neil.