I contemplate converting my old and trusty steed into a fixie.
Problem is, it has vertical drop-outs. Is there a chain tensioner that works with a fixed gear, or is it hopeless?

Nope. If you want to run fixed, you can't use a chain tensioner or else you'll get windup when you backpedal.

Here's your answer:

http://www.whiteind.com/eno.htm

see the eccentric fixed/freewheel gear hub which has an offset axle so that you can adjust chain tension. Have seen them for ~ $100 on eBay.

Also there is a site (can't find right now, Sheldon's maybe?) that shows what gear combinations will work with your chain stay length to give you whole-link chain lengths. Much more tricky, but far cheaper. Also, Sheldon has an article where he grinds a flat spot on the axle to give that little bit of play neessary to tension the chain.

TF

If your stead has reasonably long dropouts you should be able to find a gear combo that works that will get good chain tension. You may not be able to get the exact ration you want but you can generally find something close.

See http://www.peak.org/~fixin/personal/fmu/php/index.php for details about how to work out what would work in your frame.

I have old Bottecchia road frame working just great so its definitely not hopeless.

You can also use a halflink to get some gear combos.

Otherwise you will need to use one of the methods already suggested, such as ENO hub, modifying your dropouts (which should be a last resort) etc.

Good luck

the Soulcraft Convert tensioner locks into place, rather than using spring tension like the others... I've never tried it, but I was thinking that by keeping the arm locked, plus the use of a roller v. toothy pulley, it wouldn't wind up on backpedal pressure. Anyone tried it, or know why it wouldn't work?

http://www.soulcraftbikes.com/id57.htm

the Soulcraft Convert tensioner locks into place, rather than using spring tension like the others... I've never tried it, but I was thinking that by keeping the arm locked, plus the use of a roller v. toothy pulley, it wouldn't wind up on backpedal pressure. Anyone tried it, or know why it wouldn't work?

http://www.soulcraftbikes.com/id57.htm

Being solid doesn't help. The bottom of the chain is going to be straight from the chainring to the sprocket when you back pedal. I don't know what will give on a solid tensioner, it or your dereailleur hanger.

TF