Canon EOS 550D (Rebel T2i)...

[NQ6U]

Mercy ...
<sarcasm> I'd sandblast the zinc coating from both, 'vee' the slot where the washer attaches, and perform a full penetration heliarc weld using base metal with at least 80,000 lb of tensile strength. </sarcasm>

You mean TIG (Tungsten-Inert Gas) weld it, don't you? Heli-Arc is a trademark and no one uses helium anymore anyway. They've switched to argon ;)

[kf0rt]

You may be right. In either case, I'd take a grinder to the washer and flatten one edge before I brazed it in place.

I actually did that. :)

And it was a pan head. Found it in a drawer of hardware parts in the building's maintenance shop.

Highly qualified mechanical dood, I ain't. I should mention also that this screw is too long (bottoms out in my D80). 1/4" ought to be about right.

Here's the same kind of arrangement, but for 14 bucks (and used as part of a camera strap):
http://www.blackrapid.com/product/hardware/fastenr3/

[ab1ga]

If duct tape won't work, try carpenter's glue.
If that won't work, try CA glue.
If that won't work, try epoxy.
If that won't work, fix it in software.

[W3MIV]

I had a tripod years ago which I had bought from Spiratone (remember them?) that had a screw just like that at the bottom of the center post. The screw was intended to be removed and used just as your make-shift, with a length of parachute cord. On the tripod, it was intended to permit you to add a weight, a sand bag or similar bit of avoirdupois, to the bottom of the center shaft to add steadiness. I still have the tripod somewhere around here. It was too big and bulky to be suitable for 35mm work, having only three-section legs, but was good for larger formats.

I never found the screw worked that well when using a length of cord, although it did yield a stop or stop and a half at most. Under tension, it is still a bit squirrelly and the tension can make your hand shake if you try to pull to hard. If you've been humping along and try a quick catch while you are breathing hard, all bets are off. YMMV. A monopod definitely works better. My favorite is a fly-fishing staff that has a removable walnut knob under which is a 1/4" stub of 1/4-20 screw. I usually carry a multi-section monopod that collapses to about fourteen inches and a Velbon ball-head tripod with my bag. Canon's IS system seems to work well for a couple, maybe two and a half stops -- with that system, the string would really come into its own.

When working out of the back of the auto, I use an old Tamrac duffle; when I am forced to hike through the cow flops and chicken flickin's, I use a Lowepro back pack.