Talk to a hire company about magnet drops. They're mostly used for cloth, as far as I know, so I don't know how much weight you should put on them, but if you have the budget and the weight is okay they should do the job quite easily and smoothly.

Here's a crude method:

Use a scrap piece of 1x6 about the width of your I-beam (to help it be clamped in place). Screw 2, heavy-duty eye-bolts (the larger, the better) into the lumber, about 4 inches apart. Align the opening of the "eyes" so a wooden dowel (about 6-8 inches long) can slip easily between them.

Drill a tiny hole through one end of the dowel and attach as sturdy a wire as possible to the dowel. Then run the other end through 1 eye-bolt, through a loop in the shoe's shoelace, and finally through the other bolt.

The shoe will be suspended by its lace and supported between the 2 eye-bolts by the dowel.

Mount a similar 1x6 further down the I-beam with a single eye-bolt (same size as the others).   Run the free end of the wire through this and down to your stage-hand.

On cue, the stage-hand yanks the wire. The dowel will slip through the double-bolts and the shoe will drop. With practise, the dowel will lodge in the single-bolt contraption and not clatter to the stage as well.

I have done similar events several times and used an idea similar to that of Topper, but a far less complicated (and quieter) method of using a solenoid to pull the support out (with enough foam rubber padding behind the armature to make the solenoid silent) works much better.  Just make sure the solenoid has enough pulling power to remove the support reliably.  We have done this in front of an audience not more than eight feet away and they were completely surprised and unaware of how it was done.  Good luck with whatever you do.