There are a few ways to "Play" An inconsistent player.
I don't allow a player to play who does not have a background to his/her character. Motivations, Obligations, Responsibilities, and Attitudes are all important. If the player is then unable to attend, I can "Sock puppet" Their actions and responses.
If in activity becomes frequent, I will notify the player that his character is going to be NPC'd and then I find another active individual.
In the Play By Post environment, having a means to send reminders for players' turns to the respective players is crucial. I play in several other Play By Post games where the community exists outside of the game. Between emails and social media, we are kept up to date on events related to the game and those playing the game.
As a GM, I do a lot of work in keeping things interesting. If a player is interested, they tend to be more motivated to be involved.
Some players are not cut out for Play By Post as there is not as much action as a FtF. But discerning which format fits which player is tough to do with players new to Play By Post or those new to RP in general.
I like your style Malcolmshaw, penalize the ones that don't care. Having a character turn into an NPC is one way to go. JB tends to make some unlucky things happen even death.
I tend to deal with undependable players by choking them out of the story. They are always welcome to swing a sword against the monsters, but the person who only shows up on occasion finds that their background gets ignored, their actions frequently get merely summed up when they haven't bothered to post (Sometimes in ways the player may not approve of), and they basically get relegated to NPC-level - important NPCs that can still fight/ cast spells/ whatever, but not worthy of travelling with the group and participating. Give respect, get respect, I guess.
This can be curbed somewhat by how you as a Dungeon Master interact with the player, sometimes. Some people you just cannot reach if you stood over them with a crowbar and a bad attitude. But I will try to straighten out a player first by talking with them Out of Character: and explaining the expectations of the campaign and offering tips to help them adjust. I find that works wonders for some of the people who seldomly post out of frustration or confusion.
I'm in Falrun's Role-playing Game and everyone is active so dependability is not an issue. I think part of it has to do with him answering our actions so fast. So if you want dependable players then be dependable yourself. ;)
Dependable players can not only be counted on to show up when they should but to do the actions you hope they will. If some enemy is attacking my character from the back I would hope another player nearby will say his character is going to protect my back instead of doing something not so important.
Its all about communication. When you can't make it communicate it. Its the expectation that you're supposed to be there but you aren't that bothers most people.
Game Masters, I did the work of creating a Game Master's Nag Player Button, therefore if a Player is not being dependable, for instance: isn't showing up on time for an action or at all please do use it.