So, it seems that when armies are drilled, they reinforce - except if your maintenance is at 0, you don't actually pay for the reinforcements. Ironically, this applies to mercs as well, even though they don't otherwise benefit from drilling, and you don't pay drilling upkeep for them.
Some pictures to elaborate:
Armies, including mercs reinforcing during drill
Army upkeep at 0 maintenance, all getting drilled
Army upkeep at full maintenance, large reinforcement costs
Some pictures to elaborate:
Armies, including mercs reinforcing during drill
Army upkeep at 0 maintenance, all getting drilled
Army upkeep at full maintenance, large reinforcement costs