There are ways to work around this from within the plan. The files that need to be modified can be checked out prior to the stage/job that needs to modify them. For example, the teams I work with need to update a resource file with a build number (that is not the Bamboo build number). That file is checked out, incremented and checked in within the build.
Perforce is a pretty flexible platform. If people want to use it the way that is described here, it is happy to oblige. One need only define the workspace with the option "allwrite" in the client specification. See this link for details: http://www.perforce.com/perforce/doc.current/manuals/p4guide/02_config.html#1067985
I don't see any need for Bamboo to support this, because it can be defined by the Perforce user.
Thanks Bill.