If you can supply the D2Flex with the correct voltage for your setup then it will work just fine for dimming. You can do this with a resistor between the battery and the D2Flex. PWM technically is a pulsating current but it does this so fast that you shouldn't be able to visually notice it.
From the TaskLED site:
NOTE 2: The user should measure the Vf of each LED at the rated current. For example, if using four P7 LEDs, measure each Vf at 2.8A and sum the total Vf (lets assume they are 3.4, 3.5, 3.3 and 3.4 for this example, so the total is 13.6V). Then take the fully charged battery voltage (typically 4 li-ion cells) of 4.2 x 4 = 16.8V. Calculate the necessary resistor as below:
(16.8V - Total_Vf) = (16.8V - 13.6V) = 3.2V
Next we calculate the necessary resistance we need to add:
V = I x R, or R = V / I
3.2V / 2.8A = 1.2 ohms
Without adding in the 1.2 ohm resistor in series with the P7's, the initial current when the batteries are fresh off the charge will potentially be much greater than the current rating of the FET (3.4A) and the FET can be damaged.
The user interface for the dimming and on/off would through the use of a momentary pushbutton switch. So really the only extra work on your part over the MaxFlex would be the addition of an appropriate resistor.