Specifically, can I print one side of a sheet, mark it up with a standard Bic pen, and put it back in the printer to print the other side?
Paper re-inserted into a laser printer is more likely to cause jams. This gets worse the more the paper is handled, bent, crumpled at the edges, etc. It gets worse in high-humidity situations. So that's a certain reason not to do it. An uncertain reason : Laser printers rely on a very high-temperature fuser, which the pen ink wasn't designed to handle. It's likely that the pen ink will partially burn or evaporate, making a smear on your fuser, and possibly staining every printout after that. Source : Have worked in a toner refill factory.