The question is simply one of how much power per area on the lens surface. This is called irradiance, defined as power of electromagnetic radiation per unit area which is measured in watt per square meter.

To calculate this, take total power of laser but divide by the size of the cross sectional area of the beam.

A typical lens will be designed to handle the irradiance of the sun at the very least which is about 120 watts per square meter. Of course, interior components of the lens should be able to handle much higher levels as it focuses the sunlight.

What you need to find, is whether the lens can dissipate the excess heat from the inefficiency of transmission of the light. A good lens transmits quite a bit of the light, but there is always some loss. That loss can be measured in watts, and it will be concentrated in the area of the beam.

The consider the ability for glass to conduct the heat away. You must be able to guarantee that the glass can conduct the generated heat away before the glass gets to the melting temperature. Regular glass has a thermal conductivity of 1.05 but you might want to contact Canon for the specifics of the the glass they use.

Given the power of laser, the size of the beam, and thermal conductivity of the glass, you should be able to calculate whether the lens will melt or not!