Hi!
The text editor provided with PlanAhead is somewhat limited, but luckily there is an alternative: XEmacs with its famous VHDL mode: http://www.iis.ee.ethz.ch/~zimmi/emacs/vhdl-mode.html
XEmacs can be set as the default text editor in the PlanAhead Options. Unfortunately, it opens a new window each time you open a file. But even this can be solved, using the XEmacs server and client mode.
Please find a script to utilize these features at http://pastebin.com/eXmhrZiG
Please read the installation instructions in the detailed comment at its top. Note that it requires an additional step to copy-paste to a second script running in XEmacs Lisp.
It was developed and tested on a Debian GNU/Linux machine (sid) using ISE 14.2 but should also work on any other up-to-date Linux or Windows machine.
Bye
Hansi