As I mentioned earlier, simple harae and misogi rituals are an essential part of any visit to a jinja, and all matsuri. I don’t think I need special rules for them; they are something to be mentioned as colour. Shinto believes that people acquire kegare through their normal activities, and the standard harae rituals can be considered to remove that impurity. That suggests that a penalty should apply to matsuri if personae perform them without performing the standard harae rituals, but that would take a deliberate declaration by the players that the personae were not performing those rituals.
However, for kegare gained as a resource statistic, something more is needed. The personae must create and perform a special harae to purify themselves.
A harae is similar to a matsuri; there is still a norito, for example. However, the mikë is replaced with haraegu, the items to which kegare is transferred, and kagura is replaced with misogi. Unlike kagura, misogi is not something the personae can fail to perform, but it can only be included if the persona has a suitable body of water available, and is willing to get wet, so including it provides an automatic 1-step bonus, in addition to any further bonuses from the design of the misogi ritual. In a similar way, harae that can only be performed by more than one person get a bonus analogous to the bonus gained from kagura. If two people are needed, the bonus is 1 step, if four, 2 steps, and if eight, 3 steps.
Every step added to the harae ceremony purifies one point of kegare from a single persona, the persona at the focus of the ceremony. The purified persona does count as one of the people performing the ceremony, for the purposes of bonuses to the number of steps.
The most important difference between a harae and a matsuri is that a harae asks the kami of harae to remove kegare, and these kami are always both able and willing to do that. This means that there is no need to roll to see whether the kami are capable of removing the kegare, and also no need to roll to see whether the kami respond to the request.
This is a good thing, as the standard penalties to matsuri from kegare do apply. If the players needed to roll to see whether the kami responded to the request, they would almost certainly fail. However, that is not necessary, so that penalty is not an issue. The other penalty for performing matsuri while suffering from kegare does make a difference, however. That is, for every point of kegare held by a participant, including the persona to be purified, the number of steps in the harae is reduced by one. Clearly, there is no point having other people participate if they have any kegare themselves; the penalty from the kegare will outweigh the bonus from additional participants.
There is one final requirement. A harae must remove all of a persona’s kegare, or it cannot remove any of it. That is, if a persona has 6 points of kegare, she can only be purified by a 6-step (or greater) harae. Since her kegare subtracts 6 from the number of steps available, the harae itself must have at least 12 steps. With three elements, the norito, haraegu, and misogi, this is not impossible, but it is not easy.
This, then, is what provides the limit on taking kegare. If a persona acquires too much kegare, she will not be able to purify herself.
At the moment, I want to say that, while other personae can help to design the ceremony, or even do the whole design, it must be designed anew every time. That is, the personae cannot create a standard harae ceremony that they all use to get rid of their kegare. From a game design perspective, we need to strike a balance between the personae being able to take as much kegare as they want, whenever they want, and players never daring to take any. We also need to avoid the design of harae ceremonies becoming a chore that players need to get through to get on with the interesting parts. These are not easy questions, and they are ones that will be addressed in the playtest.
And that is the next step: writing a playtest scenario and running through it.