Maybe I am dense here, but I do not have a clear conception of your problem. Is your goal to entrain a certain constituent into fluid? Could you give a similiar system example if you cannot give us the actual constituents?

The jet flow from the nozzle is in the form of oxygen. It entrains air from ambient surroundings. The ratio of entrainment is approx 3.7:1.

PAC,
 
   I can suggest two things that may or may not be applicable, since I don't know the specifics of the system you're working on.  First, could the system be placed under a vacuum if it isn't already?  If not the whole system, maybe just the part downstream that is creating the backpressure.  Secondly, what about a vent valve downstream of the diffuser...it could be used in conjunction with a pressure sensor to automatically open and bleed off pressure to some type of tank, or atmosphere if the system is non-toxic, etc...

You have a mixer with your entrainment device. Your mixing variables are (see a book on chemical reactions engineering) mostly time and mixing ratio.

In your system the mixing ratio is a bit degraded with the backpressure. Therefore your primary variable is time or larger volume.

If you installed a tank or a fat section of pipe downstream of your device you would obtain better mixing. You might You might also consider positioning the nozzle in the tank or fat section of pipe such that you will get recirculation. You could look into jet tank mixers (Penberty?) to get an ideal of the spacial dimensions and mixing ratios you might expect.

Good Luck, let me know if I have missed your boat here.