To maximize long-term battery life, should I increase brightness during charging? My MacBook Pro can undergo 1000 charge cycles (using up the battery completely and charging it back to 100%) before the battery's life is reduced to 80% of what it originally was.

Obviously, using high brightness while the computer is plugged in speeds up the rate of battery discharge, causing you to plug in the computer sooner and thus use up charge cycles faster. However, if I were to increase the computer's brightness during charging, or up the energy usage in any other way, wouldn't the process of charging slow down, the time between charge cycles would increase, and thus the long-term battery life would increase?

In other words, does increasing energy usage during charging actually increase battery life?
The short answer is no, although this depends on what you mean by battery life.

You are correct that placing more of a burden on your computer while it's being charged (e.g. upping the brightness level, converting videos, etc) will result in a longer charging time (how much will differ on what your MBP is doing at the time).

However, this doesn't increase the life of the battery itself. Of course, in terms of a human measurement of time (in terms of hours/days/weeks etc) there may be a perceived difference in the life of the battery, but the battery itself isn't lasting any longer in terms of its actual usage. 

Put another way, it's common that two users buying the exact same model MBP with the exact same specs will have a different experience in terms of how long it is before they need to replace the battery. But one user may use their MBP on the road etc every day, while the other only uses it a few hours every couple of days. Or one user spends a lot of time compressing video, while the other only does basic browsing and word processing. The fact that one of these users may need to replace their battery in a couple of years doesn't mean the battery life was less, just that its battery life was used up sooner than the other.

For further info you may want to read the entirety of what Apple says about their batteries here.