acp:allowsin Link headers. You are also using it in the ACP Document, where it makes sense to have it be related to a acp:AccessPolicy
The authorization-panel is currently/generally transitioning from the Use Cases that was proposed to coming up with (Non-)Functional Requirements. The Requirements will actually be what all proposed solutions need to address as normative spec text. There is also the UC survey to determine which UCs the Community Group is interested in seeing/using/developing for servers/applications so as to help determine and prioritise which UCs the authorization-panel should focus on. At the same time, ACP is a WIP proposal addressing some of the UCs in which are not deemed to be covered by WAC. The details pertaining to delta/overlap between WAC and ACP are also being worked on. To make things even more interesting, there is the possibility of an Authorization Framework for the Solid Ecosystem which might tie up the work, allowing wider coverage of UCs by different (mostly/hopefully orthogonal) authorization systems.
So, it is all moving forward. Things should clear up in the next weeks/month(s?) As for the current Solid Ecosystem, WAC is currently a MUST for Authorization. The output of the authorization-panel will be proposed for the Ecosystem.