To maximize long-term battery life, should I increase brightness during charging?
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.