for truly fixed, it is not possible to have a chain tensioner because resisting will cause the tensioner to wind up and will slacken the upper part of the chain and cause nightmares.

If you were truly creative, you might find some way to have some kind of moveable fixed tensioner. I'm envisioning something that could be locked into place or released with a linkage of some kind. To shift rings, you would unlock the tensioner (so that it behaved like a typical spring-loaded tensioner or RD), shift the front on the fly, and then relock the tensioner to (hopefully) allow proper resisting without allowing the works to get gummed up.

A strong tensioner (steel cog on a bearing of some kind) mounted on a sturdy swinging arm could work if it had locking provisions. Perhaps a rod that goes from the tensioner arm up to the chainstay with a remotely-controllable locking mechanism to bite down on the sliding rod.

This would probably overcomplicate a fixed beyond the point of simplicity that so many like, but it just might work to allow multiple fixed gears without the rotational slop of a hacked SA hub conversion.