I.e. A pull-down list that looks like
|Custom Field Name| @Project1@|
|| @Project2@|
|| @Project3@|
|| ...|

resulting in a saved page that has

|Custom Field Name| @ProjectSelected@|

Use Case 1:
We have several development areas that benefit from being in separate projects on the same redmine database. However, some efforts involve multiple projects. Right now we have an integration project where we enter parent issues, and then add the sub-tasks needed to finish the integration. A custom list field has been created so that each subtask can identify which area (project) is involved. It would be very useful to be able to click on those items on the saved page to go to the project. This is an in lieu of having a main task that lives in a different project than the sub-tasks.

(Generic) Use Case 2:
It would be useful to be able to create a list of non-Redmine resources, any of which could be associated with an issue, so that the resource is easily accessed from the issue page.

#5794 goes into the usefulness of supporting full text formatting. That's potentially interesting, but this seems more worthy of a special case, plain or link only imposition.