Many thanks for reporting this issue. I'm resolving this issue as a duplicate of the linked issue, as the functionality requested will be covered there. Please watch that issue for updates.

Regards,
JIRA Agile Team

I have a custom field for Customer(s). Creating an Epic for each Customer isn't a feasible option, so I can't see us using the Plan mode the way it is currently as well.

Thinking further, the swimlanes for a Plan mode might not be the best solution, considering the high amount of issues to rank, and really an Epic-like solution for "custom fields" would be awesome.

Right on the spot! If that were to work for assignees, we'd certainly be able to (finally) move to the rapid board.
Components, Projects of-course and Labels would be the icing on the cake for our case, but certainly a must if you consider other teams and methods of dealing with such things.

Here's what I think they should do. Look at this rapid board. https://jira.atlassian.com/secure/RapidBoard.jspa?rapidView=96&view=planning. Notice how they've implemented Epics. I think they should build on this and have something similar for Assignees and also Components, and possibly Labels. I think this would solve all our problems.

After a semi-long discussion with my colleague (started by the innocent question "why aren't we using the rapid board again?") I decided to throw together a quick mockup that may present a potential solution to this.

Right now, the Plan Mode shows a short "How to" on the right-hand side (top part of the mockup), while selecting an issues shows details for that particular issue (bottom part).

Whats kind-of missing is the middle part of the mockup, which shows some sort of summary for the selected sprint - this should be similar to the classic Planning Board view and list issues by type and/or by assignee (maybe configurable to use an aggregated time estimate, story points, or simply the amount of issues per assignee).
As alternative, I could also imagine having a merged view of top and middle parts, where the Plan Mode introduction is replaced with the first upcoming sprint summary (at least for our cases, we may start planning a second sprint to follow, but it really gets interresting to see the workload once it becomes the next upcoming sprint).

We too have a need to be able to see workload by assignee in plan mode. Our only current option is to create a quick filter for each team member

Another idea on this:
The right side of Plan Mode currently shows the steps on what to do here. I'm pretty sure this thing could be moved elsewhere, and be replaced with some sort of summary view.
It could, for example, list all Assignees (or Components) along with the amount of issues (or the sum of estimated time, the sum of story points/business value, etc) as quick indication of how issues are distributed in the ready-to-start sprint.

Aww, and then we thought this is gonna work.
The last line above unfortunately doesnt always work that well. We just had the case that more issues were around than issues assigned to the version. The Quick Filter hides them, but they are still added to the sprint. Only resetting them afterwards using Bulk edit on the Sprint Field does the trick in such cases, so a built-in solution to the Rapid Board would be really great.