"Hey guys, here's how I break that rule I don't agree with and here's how you can do it to."
The character minimum was put in place to improve the quality of post content. Nothing of much substance that can be posted should fall below that limit anyhow. "Yes.", "LOL", "I agree.", "What he said", "x2" or similar can pretty much all be conveyed with the Like button on a relevant post. "No.", "You're wrong", "I disagree", "GTFO" all bring no actual value to a thread's conversation and can/should be better thought out with just a few more words.
Some people might see the character minimum kinda like speed limits, "Sure, it's technically 55mph, but screw it, that's slow. I'll do 65." A rule that's clearly outlined, but can be bent when we feel like it. But, no, that's not how it works.
When the forum was created, the 20-character policy was a step to help ensure a certain level of post quality, and post quality is what matters (to Admins, active members, and lurkers).
Trying to use loopholes like the underscore or literally typing "20 char limit" at the end of posts instead of spending a handful of seconds to think of five whole words you want to type will not fly. Breaking that forum policy can result in the post being deleted or, actually, a one-week suspension of posting privileges if the issue continues.