Unfortunately, the DAC can not use the AREF voltage. This is a hardware limitation inside the chip, so there is no workaround.
All the hardware details are in the
reference manual. Here's the DAC's block diagram, from page 730:
(click for full size)
As you can see, the DAC has only 2 options, DACREF_2 and DACREF_1. The connection of these 2 internal signals is documented on page 107:
The chip simply can't use the AREF pin which connects to VREFH. See the
schematic for details.
Your only options are 3.3V VDDA or 1.2V internal reference, which are selected by analogReference(EXTERNAL) and analogReference(INTERNAL).
For the ADC, analogReference(EXTERNAL) uses VREFH, which "defaults" to 3.3V due to the 470 ohm resistor. If you apply a reference voltage to AREF, it is used by the ADC, but not by the DAC. Unfortunately, this is simply the way Freescale made the chip, so there's nothing the Teensyduino software can do to give you DAC output based on your 2.5V reference.