overvolting

Ok, what can you get away with? Are there any tips for knowing what will work and what'll go fizzle?

Standard rule is 1.5 times nominal voltage. But certain motors will get away with much more like the bosch 750 I believe.

The 1.5 rule is a guideline, not a law of nature.

First of all. If you drive up the voltage, you drive up the amperes too. And that equal to the increase of voltage.
1.5 times voltage means 1.5 times the amperage. Increasing the total power with 1.5*1.5=2.25 times

Problem is efficiency. Imagine a motor is 80% efficient. This means 80% of the power becomes motion, and 20% heat.
Drawing 100 Watts means you're making 20 watts of heat. That's a weak soldering iron. Drawing 1000 Watts, and you have 200 watt of heat. That's a humongous soldering iron.
Of course, that's when a motor is used at it's best running conditions (the very very top best of electric motors running at the best conditions will convert 95% of the electric power into movement) . In stall (jammed drive) the efficiency is 0. no movement, all heat.
See the issue there? With great power comes great heating....

The example, a Bosch GPA750 24V can handle overvolting to 36V, but after "ruggedizing".
Welding the crimped in front plate is just to avoid shock damage. And can be avoided by clamping the body or supporting the backplate.
But glueing and/or mechanicaly locking the magnets is mandated, as the higher torque will have the magnets crush the feeble springs that hold them in place.

The much heavier per watt wheelchair motors can handle overvolting a tad better without extensive changes.

Ok, that's all more than handy to know.

Are there ever issues with the current skipping across gaps at higher voltages? ie, insulation is not sufficient for the new current?

Never seen any issues with that.

Unless you have a cooked motor, then the windings will short out due melted insulation.

Ok lovely, tah :)